I kind of go for simple and clean. I was using "Daum Blue" for a while, but "Mangan" updated recently so I switched back to that. There's also "Just Dark" which is a more monochromatic take on the Firefox Default theme, and "Unofficial Dublin," neither of which I could use at the moment since they haven't been updated to compatibility with my current install of Firefox 3.5.2.
It's great that there are so many themes available, it seems like there's something for everyone.