It depends on what games you'll be playing. I prefer MTT/SNG nlhe.
1st - Fulltilt (by far more than the rest)
2 - Pokerstars
(I guess.... but rarely play on there)
3 - Doyles or Titan (don't particularly like the software of either, although Titan has improved their's ALOT just lately..... I like to play MTT's & on both of these sites the field sizes are a bit too small for my liking (ie. $10-$30 buyins, ~ 100-200 entrants,... I'd rather there be 500-1,000 entrants)