I change sites a lot since I tend to play at one site for a month, get fed up, cash out and then not play for 3 months. After which I inevitably don't want to return to the same site. Most recently I was on
Irish Eyes Poker and
PKR.
Irish eyes is a good site with excellent text file logging so you can review hands with ease at your convenience. The client itself is pretty basic but there are a wide range of ring games/tournaments/freerolls etc.
PKR is, as far I know, the most graphically advanced poker client. It lokks awesome when compared to the others and has the broadest range of emoticons and shit. These just annoy me after 5 minutes so I turn inevitably them off but some people really like it. The choice of games is a little more basic than at Irish Eyes but it's still a pretty good site.
All that said, I'm not convinced either of these sites is the best I've ever played on although PKR would be one of my most successful.