RAM Use

Below is a graph showing the RAM use in excess of the 261 MB used by the base system (without GDM3 or X.Org running).

This is based on 3 test runs:

• Run 1:
• separate Ubuntu distributions/"flavours" in separate VMs (official versions: Kubuntu, Xubuntu, Lubuntu, Ubuntu Budgie, Ubuntu MATE, Ubuntu Kylin, elementary OS; inofficial versions: Unity Remix, Lumina Remix, Cinnamon Remix, Ubuntu DDE; Ubuntu Net Install with manual installation of Gnome-Shell, TDE, Enlightenment, and CDE)
• RAM measured with screenfetch
• Run 2:
• all DEs installed in same system with different user accounts (1 DE = 1 user) to keep settings separate
• vanilla desktop settings (e.g. standard Xfce-Session instead of Xubuntu's version)
• RAM measured with screenfetch
• each DE was measured after logging in and out of another DE (this lead to higher RAM use on average, possibly because some processes and services were not killed after logging out the previous user)
• Run 3:
• all DEs installed in same system with different user accounts (1 DE = 1 user) to keep settings separate
• vanilla desktops (e.g. standard Xfce-Session instead of Xubuntu's version)
• RAM measured with top
• system restart between each uptime

Results:

1. Enlightenment: 70 MB. (best performance: run 3, Ubuntu 20.04 + Enlightenment from Ubuntu repository)
2. Lumina: 93 MB. (best performance: run 1, Ubuntu 20.04 Lumina Remix)
3. Trinity: 185 MB. (best performance: run 1, Ubuntu 20.04 + Trinity from developer's PPA)
4. LXQt: 243 MB. (best performance: run 1, Lubuntu 20.04)
5. Pantheon: 302 MB. (best performance: run 1, elementary OS Hera based on Ubuntu 18.04)
6. Unity: 335 MB. (best performance: run 3, Ubuntu 20.04 + Unity from Ubuntu repository)
7. Budgie: 336 MB. (best performance: run 3, Ubuntu 20.04 + Budgie from Ubuntu repository)
8. KDE Plasma: 347 MB. (best performance: run 1, Kubuntu 20.04)
9. Cinnamon: 348 MB. (best performance: run 3, Ubuntu 20.04 + Cinnamon from Ubuntu repository)
10. MATE: 376 MB. (best performance: run 3, Ubuntu 20.04 + MATE from Ubuntu repository)
11. Xfce: 394 MB. (best performance: run 1, Xubuntu 20.04)
12. GNOME: 408 MB. (best performance: run 3, Ubuntu 20.04 + Gnome-Shell from Ubuntu repository)
13. CDE: 419 MB. (best performance: run 1, Ubuntu 20.04 + CDE from SparkyLinux repository)
14. UKUI: 438 MB. (best performance: run 3, Ubuntu 20.04 + UKUI from Ubuntu repository)
15. Deepin: 490 MB. (best performance: run 3, Ubuntu 20.04 + Deepin from UbuntuDDE PPA)

Having a closer look

Budgie

• Desktop environment originally created for Solus but also available for other distributions
• Based on GNOME 3, doesn't come with own applications but uses GNOME apps
• More traditional and convenient workflow than GNOME, with dock showing opened apps
• High RAM usage, like GNOME
• Elegant design
• Conclusion: average
• Features: 3/5
• Polish: 4/5
• Resource usage: 2/5

Common Desktop Environment (CDE)

• The oldest desktop environment tested, created in 1993 for commercial Unix systems, later ported to Linux, open source since 2012
• Uses the Motif toolkit, unlike most other desktops which use either GTK or Qt
• Somewhat inconvenient workflow (no window list, no window snapping/tiling etc), some quirky ideas (from today's perspective), such as minimizing windows into a desktop icon
• Medium RAM usage (surprising as this desktop is straight from the 90s - maybe it's not optimized to run on Ubuntu yet)
• Very classic design, unique feel
• Not quite recommendable as a daily driver, but if you like the look and feel there is nsCDE
• Conclusion: not recommended
• Features: 1/5
• Polish: 2/5
• Resource usage: 1/5

Cinnamon

• A more traditional interface alternative to GNOME 3, from which Cinnamon was forked
• Created by the Linux Mint team and their flagship desktop environment
• Uses mostly GNOME/MATE applications
• Traditional interface with start menu, taskbar and tray
• Quite customizable, definitely moreso than GNOME
• Better file manager than GNOME's "Files"
• Average design, neither very modern nor very old fashioned (the above screenshot is from Ubuntu Cinnamon Remix and uses an ugly orange theme - Linux Mint looks better)
• High RAM usage, like GNOME 3, is the main drawback
• Conclusion: recommended
• Features: 4/5
• Polish: 4/5
• Resource usage: 2/5

Deepin Desktop Environment (DDE)

• Created for Deepin Linux, a Chinese Linux distribution
• Based on DTK (fork of Qt), comes with its own set of applications
• Elegant, modern design
• Easy to use and pretty, main downside is that you have to trust the Chinese developers to not have put any hidden CCP spyware in
• Medium-high RAM usage
• Conclusion: average
• Features: 3/5
• Polish: 5/5
• Resource usage: 1/5

Enlightenment

• Started as a window manager in the 1990s, now it has slowly grown into a small, lightweight desktop environment, with its own set of core applications
• Uses it's own widget toolkit, which is also used in Samsung's Tizen operating system
• Customizable and powerful window manager, rather basic file manager
• Very lightweight compared to all other desktop environments, however not so much when compared to stand-alone window managers (e.g. JWM, IceWM)
• Conclusion: average
• Features: 2/5
• Polish: 3/5
• Resource usage: 5/5

GNOME

• Default for virtually all corporate distributions (e.g. Fedora/RHEL, Ubuntu, Solaris), and generally available for most distributions
• Backed by IBM; many of GNOME's core developers are IBM (Red Hat) employees. Thanks to this, GNOME feels quite polished, without obvious bugs or papercuts.
• Back in the 90s, GNOME was founded because KDE used the (then) proprietary Qt toolkit, and GNOME was hence built on the completely free GTK toolkit created for GIMP. Shortly after, Qt became free software as well, but GNOME development continued. The KDE-GNOME schism remains until today.
• Together with KDE, GNOME is the only desktop environment will a full set of applications
• GNOME 3 was a very controversial release, with a completely new UI and workflow, which was mainly targeted at tablets and touchscreen devices, because everyone in 2011 thought PCs would die and everyone would use tablets in the future (see also: Windows 8). While Windows went back to a more traditional interface in 2015 (Windows 10), GNOME has retained its idiosyncratic workflow.
• There is no taskbar or dock indicating running applications or open windows. In fact, in order to switch windows or open a new application, one needs click in the top left corner or use the Super key to access the "Activities" screen, a fullscreen overlay showing thumbnails of all open windows as well as a button to show the list of installed programs. This hassle to switch windows encourages users to display only one app at a time (just how you would on a mobile device running iOS/Android), hence also the lack of minimize and maximize buttons.
• Nautilus, the file manager, lacks some very elementary features, such as type-ahead find or thumbnails in the file picker.
• GNOME is probably the least customizable desktop, there are very few available settings, no theme API etc.
• High RAM usage compared to KDE Plasma or Xfce
• Average design, neither very modern nor very old-fashioned
• Regarding the GNOME developers:
• The GNOME developers have shown a disregard for user feedback, for example, it has been over 15 years and the GNOME filepicker still does not support thumbnails, or as another example, type-ahead file selection in the GNOME file manager was removed and will not come back as the GNOME developers prefer users to follow the GNOME way instead of GNOME following the user's workflow preferences.
• GNOME does not integrate well with other desktops, for example it doesn't support tray icons or server-side window buttons and has asked software developers to change their applications to follow the GNOME way, even if this would break features on other desktops. For this reason, the developer of well-known media player MPV has even accused the GNOME developers of deliberately sabotaging the Linux desktop.
• The GNOME Code of Conduct takes a stand against racism, sexism, transphobia etc, but on the other hand explicitly tolerate anti-white racism, misandry etc.
• The GNOME foundation signed the open letter against Richard M. Stallman and the Free Software Foundation in 2021.
• Conclusion: not recommended
• Features: 2/5
• Polish: 5/5
• Resource usage: 1/5

KDE Plasma

• The second-oldest full desktop environment for Linux (after Xfce)
• Uses the Qt toolkit
• Together with GNOME, it is the only desktop with a full set of applications
• Very customizable. The most features and customization options of all desktop environments.
• KDE Plasma can do anything you like, but due to the infinite amount of options and settings there is a higher chance of encountering bugs if you stray too far from the standard settings. (Although newer versions have far fewer bugs than Plasma used to have in the past.)
• Dolphin is the best file manager among all desktops. It even supports thumbnails in the file picker!
• Low to medium RAM usage, in my test it was even lower than Xfce's.
• The design was very modern when it first came out (no wonder Windows copied them), but is starting to feel a bit dated. Fortunately KDE can be themed any way you like.
• Overall the best desktop environment, with the most features, most customization options, and one of the lowest RAM usages.
• Most Linux distributions as well as BSD offer KDE Plasma. However, none of the major distributions commits to KDE Plasma as the default desktop. Historically KDE-focused distributions have either disappeared (Mandriva, Mepis) or have become desktop-agnostic (OpenSuse).
• Conclusion: recommended
• Features: 5/5
• Polish: 4/5
• Resource usage: 2/5

Lumina

• Originally developed for TrueOS (a FreeBSD fork), then for Linux-based distributino "Project Trident" which has now also been discontinued.
• Lightweight Qt-based desktop developed from scratch
• Few own applications, and they are not that great either.
• Default design is not very pretty
• The main merit is the low resource usage
• Few distributions offer Lumina
• Conclusion: not recommended
• Features: 2/5
• Polish: 2/5
• Resource usage: 5/5

LXQt

• Lightweight Qt-based desktop environment
• Successor of LXDE (GTK-based) and Razor-qt
• Few own applications
• Traditional interface with panel and the usual customization options
• Low RAM usage, interesting alternative to the more Xfce or MATE
• Conclusion: recommended
• Features: 4/5
• Polish: 4/5
• Resource usage: 3/5

MATE

• Continuation of GNOME 2
• Easy to use, flexible interface, somewhat outdated design
• Medium RAM usage
• Uses its own set of GNOME-based applications from the GNOME 2 stack
• Available for most distributions, as well as all BSDs and Illumos
• Conclusion: recommended
• Features: 4/5
• Polish: 4/5
• Resource usage: 2/5

Pantheon

• Created for elementary OS, but also available for other distributions
• Based on GNOME 3, but comes with its own set of core applications developed from scratch
• Elegant, simple design, although clearly inspired by macOS
• Not very customizability, little features that set it apart
• Surprisingly moderate RAM usage
• Conclusion: average
• Features: 3/5
• Polish: 5/5
• Resource usage: 2/5

Trinity Desktop Environment (TDE)

• Continuation of KDE 3.5
• Comes with a full set of applications inherited from KDE 3.5
• Uses TQt, a fork of Qt 3
• Low RAM usage, making it a good option for old computers
• Like KDE, it is full of features and can be completely customized
• Outdated default design, but can be themed
• Small developer team
• "Finished product", don't expect any major features added in updates
• Few distributions offer TDE, however the TDE developers offer binaries for all major distributions. A notable distribution with TDE support is Q4OS.
• Conclusion: recommended
• Features: 5/5
• Polish: 3/5
• Resource usage: 4/5

UKUI (Ubuntu Kylin User Interface)

• Created for Ubuntu Kylin, the Ubuntu variant for the Chinese market, and only available for few other distributions
• Originally based on MATE (GNOME 2), however recently switched to Qt and KWin (KDE's window manager). 
• Very few UKUI applications, mostly employs MATE applications.
• Elegant design
• Classic layout, the panel clearly imitated Windows 10
• Conclusion: average
• Features: 3/5
• Polish: 4/5
• Resource usage: 1/5

UnityX

• Unity was created by Canonical for Ubuntu, originally as an interface for netbooks, then used as the default Ubuntu desktop from 2011 to 2017.
• Canonical has since stopped further development. Their last stable version, Unity 7, is now a mature product, polished and without major bugs, while Unity 8 (based on Qt, for both mobile devices and desktops) was still very experimental when Canonical stopped development. The development of Unity 7 continues as UnityX, and Unity 8 for Linux phones has been renamed to Lomiri. Both are developed by the community now.
• Uses a global menu by default, similar to macOS
• Easy to use and the most important customization options are there
• No own applications, uses GNOME apps
• Default design is becoming a bit outdated
• Only really available for Ubuntu, no other distribution has bothered to support Unity.
• Conclusion: average
• Features: 3/5
• Polish: 5/5
• Resource usage: 2/5

Xfce

• The oldest desktop environment developed for Linux, slightly preceding KDE and GNOME.
• Traditionally the lightweight alternative to GNOME and KDE Plasma
• Mature product with relatively slow development pace
• However, recently KDE Plasma has become just as lightweight (in terms of RAM use) as Xfce, and some new desktops like LXQt are even more lightweight than those two
• Comes with a small set of Xfce applications
• Customizable and easy to use
• Traditional workflow
• Default design looks outdated, but can be themed
• Conclusion: recommended
• Features: 4/5
• Polish: 5/5
• Resource usage: 2/5