I used Xubuntu for awhile, mostly because I loved its simplicity and its black and royal blue color scheme. "OMG! Ubuntu!"'s benchmarks, though, suggest that it's basically just as fast as normal Ubuntu.

I personally use Ubuntu with the Elementary-OS theme and addons. I love how simple and uncluttered it is. It loads fast, and it's responsive both in the sense of being quick and in the sense that it feels much less "noisy" than Windows. It's like a good pet, and does what it's told.