Ubuntu, as we know, is by far the most popular desktop and general purpose Linux distribution. Flawed as it is, its here to stay. It does have its advantages though - mainly relative ease of use and "just werks out of the box". It also has good documentation (for the most part...) and a wide range of third party support in the case it breaks, however archaic its breakage is relative to the rest of the Linux ecosystem.
Granted, however, for desktop purposes there's more than just the official image. There are also community "flavours" that are spin-off images with their own desktop environments, default apps, and configurations as determined by the community that develops them. Chances are when you use Ubuntu you'll likely end up using one of these flavours - they're a great way to use a different DE for testing or for general purpose usage.
A lot of these flavours don't really fix the issues that Ubuntu has - bloat, slow to update, requiring to hunt the internet for PPAs, some bad defaults, etc. Some do mitigate these issues, while others only make it worse. That's why this article exists - there's more at play here than just desktop environment.
As for the machines used, I have my list of machines on the front page of my website. Of these machines, I used the z50-70, the Asus E403SA, and the Dell Vostro 1520, in order of strongest to weakest machine. I mostly used the E403SA for this though - it has 4 GB of RAM and a Pentium N3700 (dual-core, 4 threads) @ 1.6000 GHz (2.4000 GHz max speed) so it's absolutely a weak machine. It's the only one of my machines to have run all of these OSes. All of them ran pretty well so it should serve as proof that Linux is the true lightweight OS (other htan OpenBSD...). It ran better than M$ Cringedows anyways...
As for OS version, these are all being written off of 20.04. That's the latest LTS, and over 90% of users are gonna be using LTS. Point releases imo are better, because they keep almost all of the stability while having more up to date packages, but more software supports LTS versions, and some point releases can have some pretty significant and sometimes unwanted changes (most notably 21.04 of the main release using Wayland, which is quite a big change no matter whether you like it or x.org more, another example is Xubuntu 21.04 switching to Xfce 4.16 which brings CSD). I am not significantly modifying the OS outside of maybe changing the layout to be a bit more traditional (most notably in Ubuntu MATE). I do keep the original layout in mind for the sake of the review though and I did give the original layout a chance.
Mainline Ubuntu is naturally gonna be the starter for this, and for good reason - it's honestly what at least 50% of Linux users out there are running. As they describe it, "The open source Ubuntu desktop operating system powers millions of PCs and laptops around the world.".
As for the OS itself... it's really nothing special - just a simple GNOME stack on a heavily modified fork of Debian. The GNOME implementation, however, is probably the best of any available GNOME implementations. It's opinionated in that it takes a more MacOS-like metaphor, but this imo is a good thing compared to the abomination that is defualt GNOME. With GNOME comes good default apps, nice aesthetics, and really a lack of customization or much past that. If you don't like GNOME, go towards the flavours.
As for resource usage... last I ran Ubuntu 20.04 on my dads laptop it used around 1 GB of RAM on an unedited boot. That, of course, is with all the bloat Canonical has added. With this being the flagship Ubuntu distro, it will have all of Canonical's shitty additions such as Snaps etc.. Compared to the community-led flavours, this really sucks. But if you get rid of them, it goes down to 650 MB of RAM which is frankly not bad. It's still not the lightest, however, and it can't be slapped onto just any PC. My old Asus Vivobook E403SA could barely handle it. Given an Intel Core processor or an AMD Bulldozer/Ryzen with at least 4 GB of RAM, however, it'll be just fine.
Overall, Ubuntu is probably the best GNOME implementation out there. It is opinionated, no doubt, but in a good way - it has more sane defaults than any other GNOME-default distro out there, particularly with having the dock actaully stay out. If you like GNOME and its vision, then Ubuntu is a way to go. Vanilla Ubuntu, hwoever, usually tends to include Canonical's more corporate decisions, usually for the worse. If you'd prefer a more community-led experience, or if you'd rather not mess around with Snaps and whatnot, or if you dislike GNOME, then look for a flavour. If you can put up with those, however, it is a fine choice. Just be sure your machine can handle it, cause while it is lighter than it was before, its not the lightest still.
Lubuntu uses the LXQt desktop environment (DE). As they describe it in their manual, "The project’s goal is to provide a lightweight yet functional Linux distribution based on a rock-solid Ubuntu base. Lubuntu provides a simple but modern and powerful graphical user interface, and comes with a wide variety of applications so you can browse, email, chat, play, and be productive. Lubuntu was formerly a distribution for low-end hardware, but we have refocused."
I've written a review on LXQt and you can read that here. Since I wrote it, however, my opinion of it has gone down a bit. It does work if you want a basic Windows XP experience that just works with zero fuss, but it does have some bad defaults (not suspending on laptop close??? wtf). If you want more, however, you're not getting it. Either you work with the OpenBox config files to change keyboard shortcuts (which is atrocious, a desktop environment should abstract this away a bit better than LXQt does) or you just mess with what you have and nothing more.
It does come with a few apps out of the box, but unfortunatley... this is actually its greatest weakness. The apps are pretty good actually, but it relies on KDE apps which are heavily bloated. The only other alternative in the Qt verse is to just not use Qt and use GTK apps, but that gets bloated too with the different libraries and dependencies for each toolkit.
Also endemic to the Qt verse is a lack of themeing, and unlike KDE there is no solving this. It will look like how it does out of the box, with very little room for change. And the apps? You gotta use Kvantum for that, and it leads to an inconsistent look between GTK and Qt apps.
In terms of resource usage, it's actually pretty good. It used 350 MB of RAM from free -ht and about 450 from neofetch after debloating. If you read later on, however, Xubuntu is not far from this, and Xubuntu offers quite a bit more functionality than this does. The difference can be enough to revive a PC however and it definitely will run smoother than Xubuntu will.
In conclusion... Lubuntu is just one of the weaker flavours. It's certainly far from bad - it is uber lightweight and is the best you're gonna get in terms of lightness out of an Ubuntu based distro (unless you go for server and use a standalone WM), in particular on RAM and CPU. That being said, I'd argue that its not lighter than Xubuntu by enough to where it's worth it unless you really need the additional lightness or if Xubuntu is too buggy for you. It's rather limited due to its inherent reliance on Qt meaning that you choose between GTK bloating up in terms of dependencies or the equally bloated KDE apps. It otherwise lags behind in terms of functionality, customization and aesthetics while not being lighter enough to where its worth it. It's a good shot if you have a super weak machine and still want the Ubuntu experience, but if you can afford the additional resources, try to look elsewhere.
Kubuntu uses the KDE Plasma DE. As they describe it, "Kubuntu is an operating system built by a worldwide community of developers, testers, supporters and translators. Kubuntu is a free, complete, and open-source alternative to Microsoft Windows and Mac OS X which contains everything you need to work, play, or share. Kubuntu unites Ubuntu with KDE and the fabulous Plasma desktop, bringing you a full set of applications... Built using the Qt toolkit, Kubuntu is fast, slick and beautiful. Kubuntu is mobile-ready, enabling easy integration between your PC desktop and phone or tablet. Simply use the Google Play store to install KDE Connect on your Android device and you can integrate your device with your desktop."
Their description aside, I'll be very up front with this one - Kubuntu is likely the best Ubuntu flavour. It simply is superior to the others. KDE Plasma in itself is pretty good - it is fast, it has become rather lightweight over time, and it offers a great experience and integration. It ran pretty well on my Asus E403SA - not the fastest but seeing that KDE used ot have a reputation for being heavy, its a far cry from it. It used less than 500 MB of RAM too, which is absolutely amazing for what it offers. It will be a bit meaner on the CPU though.
In terms of everything else... in general, Kubuntu really doesn't change much, for better and for worse. And that is probably its best strength - it offers a lightweight, stable, configurable to hell and back desktop. KDE Plasma is really really good. There's really not much to say about it, until you experience how it is. It easily surpasses any proprietary offerings at their very own game... including bloat, which is its greatest weakness. No doubt about it, Kubuntu is easily the most bloated of the Ubuntu flavours.
Overall, Kubuntu just feels right. Seeing that you're on Ubuntu you're probably used to bloat/are willing to deal with it, and if you are, you'll find Kubuntu makes the best use of what it has. It's the Windows of Linux in the most postiive way possible - it has a great desktop metaphor, it just works, and it offers great functionality and apps. Compared to its competitors it outshines them on most fields.
Ubuntu MATE is a flavour of Ubuntu that uses the MATE desktop. As they describe it, "Ubuntu MATE is a stable, easy-to-use operating system with a configurable desktop environment. It is ideal for those who want the most out of their computers and prefer a traditional desktop metaphor. With modest hardware requirements it is suitable for modern workstations, single board computers and older hardware alike. Ubuntu MATE makes modern computers fast and old computers usable."
As a flavour, however, Ubuntu MATE actually does make more changes to the overall desktop experience than any other. Most notably, they add a software store that... actually WORKS. AND IS GOOD. This is a rarity that truly must be celebrated... until you realize its a Snap package. Man... its honestly good enough to where inexperienced users may be advised by me to not get rid of Snaps purely to keep it though. It also adds great themeing to the MATE desktop that truly brings back the retro Ubuntu vibe that was lost past 18.04. EDIT: I hear 21.04 is doing away with this for some god forsaken reason, but this is a 20.04 review so that's irrelevant.
As for MATE itself, I've written about this, and you can find it here. Tl;dr for those who want a quick rundown - it's one of the best DEs. It's lightweight, easily customizable for the day-to-day user, and it has great default apps and a nice aesthetic. Ubuntu MATE truly shines in this. It keeps the MATE experience intact while its additions genuinely serve to make it stronger.
The only real thing to criticize here is that Ubuntu MATE does not keep as light as it should be. On my Dell Vostro 1520 with Arc Darkest Solarized, it used 523 MB of RAM on a fresh boot using free -ht. This is still pretty lightweight, and it ran quite well on the Dell Vostro 1520 which for today is absolutely a potato machine (Core 2 Duo, 4 GB of RAM). However, it's not as light as Xubuntu, which is more the fault of Ubuntu MATE rather htan MATE itself. MATE on Arch Linux, for instance, is actually lighter than Xfce from my tests, using 286 MB of RAM compared to 316 MB of RAM for Xfce. This means that Ubuntu MATE will work on almost any machine, but it won't be quite as resouce-conserving as Xubuntu and Lubuntu.
Overall, Ubuntu MATE is a strong experience. MATE is imo the best desktop environment and Ubuntu MATE actually adds to it in a meaningful way that makes this distro perfect for almost any usecase. This does end up coming at the cost of MATE's relative lightness, however, but it still works well even on the weakest of machines. It's absolutely worth putting on any machine.
Xubuntu uses the popular Xfce DE (which, from what I've seen online seems to be the most popular DE behind GNOME). As Xubuntu puts it, "Xubuntu is an elegant and easy to use operating system. Xubuntu comes with Xfce, which is a stable, light and configurable desktop environment. Xubuntu is perfect for those who want the most out of their desktops, laptops and netbooks with a modern look and enough features for efficient, daily usage. It works well on older hardware too."
Honestly, I'm a bit biased with Xubuntu as it is what truly got me into the deep end of Linux. A lot of that genuinely does have to do with its relative lightness - out of the box it used around 470 MB of RAM with free -ht. For my Asus Vivobook E403SA with 4 GB of RAM, that's a godsend compared to regular Ubuntu 19.10 having used about 1 GB. That metric, however, is heavily weighed down by several heavyweight applications supported in the distro, most notably GNOME Software Store. Like Xfce itself, it also balloons with the wrong theme - Arc Darkest Solarized brought that 470 up to 700 MB, while Arc Dark did not. With this being said, you have to do a bit of homework if you want to truly bring it down - and it does work, a fresh boot with GNOME Software and other offenders removed + Arc Dark uses about 375 MB of RAM via free -ht and 430 via neofetch.
As for Xfce, I've written a review on it here where I voiced my opinion that it's outshined by MATE. The luster that MATE has over Xfce, however, depends a lot on them being similar weight, and as you'll read above (if you haven't already) Ubuntu MATE does not keep all of that lightness. This makes Xfce and Xubuntu honestly a lot more appealing than they otherwise would be, and puts it on equal measure with Ubuntu MATE. The Xfce implementation is rather vanilla in itself - not many changes from Xfce itself. I actually applaud Xubuntu for that. While I do appreciate the changes Ubuntu MATE makes for instance, I also appreciate keeping with the original project's vision. This is truly just Ubuntu with Xfce, no more. One thing it does do is use the Whisker Menu by default, which is absolutely the best move to do. If only it binded it to the Windows key by default...
Overall, Xubuntu is probably one of the best Ubuntu flavours. It's truly lightweight and can work on almost any machine, it looks pretty with the right theme, and it keeps the high functionality and smoothness of Xfce without changing it up too much. It does require some homework to make it truly light but if you're willing to take 20 minutes to set it up, you'll get a great experience out of it.
There are other Ubuntu flavours - official and unofficial - that have different DEs. I won't be going over them in this article as I haven't tried them out, and I'm too busy to both write the article and try them out, but you can see them here:
These are all relatively niche compared to the mainline flavours (other than Linux Mint ofc), hence why I haven't tried them out yet. I may or may not do so soon, but in the meanwhile, try them out yourself!
Overall, all of the flavours are honestly pretty good. They each have their own use cases - Lubuntu for when your machine is truly too weak to run much else but needs stability that rolling-release distros don't provide, Xubuntu when you can afford a tiny bit more for much more functionality, Ubuntu MATE and Kubuntu for all around machines, and Ubuntu if you want a MacOS-esque metaphor.
With that, I'll refrain from ranking the distros. I will say that if I absolutely had to choose one to go with over the others, I'd choose Kubuntu as its functionality and ecosystem are the best, or Ubuntu MATE if I wanted a GTK based DE. That being said, don't get too hung up on one or the other.
Finally, these projects could use your help. Please do donate, contribute, etc. to all of them.
I won't be doing a table at the end at the moment as it takes a while to format and I don't have the time to write it up, and tbf there's not much to categorize these off into. That being said, look primarily into desktop environment integration, default apps/themes, resource usage, functionality and customization. I might make a table later based on these idk.