I was always using GNOME as my default Desktop Environment in Ubuntu (which comes by default), and I have tried many others such as KDE, XFCE …etc), and I always admired GNOME the most as it has a good balance between performance and nice GUI; I found KDE to be too fancy and with slightly less performance and XFCE to be amazingly fast, but not really good-looking, and YES! I like my desktop to be good-looking, I guess anyone who lives for 16 hours daily in a room will make it as much beautiful as he/she can :).

With XFCE 4.6 (which comes by default in Xubuntu Jaunty), I found a much nicer GUI than old XFCE releases and still the same performance boost, I guess you gotta try it out.