Fulltilt is the best all around site... Easy to follow the action (meaning u can multitask and still know whats going on), Great software, tons of players and games to chose from, great lay out, and fairly good players for the most part.
PokerStars is a pretty decent site... not as developed as Full Tilt i dont think, but still a fun site to play on, dont have any complaints about it...
Bodog for freerolls only....
Absolute... is alright, but id perfer PS or FT for all games
thats all i've played on... hope that helps