Voted for XFCE and LXDE as both are fast. Before GNOME3 though, GNOME2 was main desktop environment. I tried GNOME3 and Cinnamon for a few months but can't really get the hang of it. Currently using Mint Lisa GNOME 64bit with GNOME3 fallback but I have to say, I'm still not much used to it. Can't make it work how I made my setup of the older settings of GNOME2 with MintMenu
I also use LMDE XFCE 64bit in another partition but I haven't used that partition in awhile. I'll probably do a complete reformat of my laptop once Mint Maya LXDE is out. Will add LMDE XFCE 64bit in another partition as I love XFCE eyecandy. Can't help but also need to install Winblows XP in my laptop sadly because I need those Avisynth, VirtualDub, Adobe AfterEffects and meGUI (never gotten used to Winblows 7 nor the dreaded Winbows Vista - Winblows XP with only the same partition size of the LMDE XFCE and Mint LXDE would be fine - unlike Winblows 7 and Winblows Vista which you have to have like 4 times the recommended partition diskspace). Sadly, I can only say goodbye to Winblows once I get around mastering video encoding using Linux (which I still need to give a lot of time to study).
And oh please, sorry if it's somewhat a request but - Kindly release an Ubuntu based XFCE again, pretty please~ Even if the awesome Mint team would release them during Ubuntu LTS releases - please~ X3b