I'm going to have to go with GNOME 2 (Now forked as MATE) and the Cinnamon desktop for Linux Mint.
At first I didn't like Cinnamon because it was still young and in the early stages as well as being nothing more than a glorified, touched-up GNOME 3.... At first. It has since come a long way and I now use it over MATE. No hate on MATE, though. (Unintentional rhyme, ha ha ha!)
My reasoning for liking and using them is that I feel they are the most simple ones to use as well as being user-friendly and intuitive. In my opinion, they are also pretty fast and haven't quite given me any problems so far. Any ones I have encountered were easily fixed and/or tweaked.
So what's your favorite Desktop Environment to use on Linux and why?