I have a long history with Linux, I started on 1995. When on 2013 GNOME moved to its 3.x version & Ubuntu decided to use as desktop Unity (instead of GNOME 2.x as it did before) y felt betrayed (not just me, but the whole community using GNOME).
- GNOME 2.x
- MATE (It is a live and kicking fork of GNOME 2.x)
- GNOME 3.x (and its shells)
- KDE 3.5 and below
- KDE 4.x and above (Used by Kubuntu)
- Unity (Used by default in Ubuntu)
- LXDE (Used by default in Debian)
- Cinnamon (Main Linux Mint desktop)
- GNOME fallback (Similar to GNOME 2.x)
- KWin: Used by KDE, and trying to break free from KDE dependencies.
- Compiz: This was an independent effort, then it's main developer was enlisted by ubuntu but it's now adrift
- Openbox: This is a classic, simple and solid window manager.
- Debian: My favorite! It could be a little harsh with you some times, but I still like it. I love it's open spirit, resilency...
- Ubuntu: I did like it more before Unity, but in any case it is still a recognized leader. With its "clones" Kubuntu and Lubuntu) it still has a nice side to it.
- LinuxMint: If you ask me this was the only winner with Ubuntu's move towards Unity. It has different desktops by default. The two that I value the most from them are:
- Cinnamon: This one will make you feel like home if you come from Windows
- MATE: A gateway from GNOME 3.x, will let you keep running GNOME 2.x and still have security updates.
- With RedHat move to corporate we got Fedora and CentOS as their open-source options.
- And a couple really small: Slax (218MB w/KDE), Puppy Linux (161 MB) and DSL (50MB)
To be honest I've always been annoyed by being pushed into changing my desktop. I did not like that when I used windows, and I felt like using Linux was a way to avoid that. Even windows always had a way to fallback.
On the other hand I've always chosen GNOME over KDE due to being less complex, open from the start (I refer to QT libraries different licenses). I used GNOME for a long time, and when I needed something lighter I would use Openbox instead. But Openbox is not really a desktop, so I always felt more comfortable on GNOME.
I also resisted KDE (and the QT libraries) for a long time. But my job as a teacher and the movement from GNOME 2.x to GNOME 3.x left me stranded. I tried using LXDE and I must say I like it. But on the end I felt like I wanted to have a MacOSX machine instead of Linux.
Finally I decided to try to configure KDE so it would work the way I wanted. To do that I had to find a way to avoid the Plasma Desktop, it turned out to be really simple to do that. I end up finding myself charmed by a VERY flexible, strong and stable system. Thanks to current machines speed and memory it does run smoothly and I managed to configure it so it would like as close as it can get to what I was used to. I no longer feel I need a MacOSX machine (in part thanks to KWin windows manager) and I found myself using a very stable/mature system that has an stable version within Debian repositories.
I still love LXDE and Openbox as light weight options. If I weren't so fond of Debian I guess Linux Mint running MATE would a good choice... But I finally got a Desktop setup I feel comfortable with: KDE. It takes a little of work, but it is worth it. I must accept that KDE has a lot of advantages and they are much more respecful of their community.
2015 Jan update - While reading an article about most used software on the Linux community I realized there is another BIG contestant that I did not include in this list: Xfce (A light weigth widely use by the community). I tried it and it looks good, but it will need more testing and I doubt it will be as flexible as KDE. For what I've seen and read it seems to be on the same bucket as LXDE: Lightweight, good support, limited features.
2020 April update - Found a very good article which focus on used memory: A Memory Comparison of Light Linux Desktops