I've played a few sites, and my order of preference is...
1. pokerstars - best interface, best customer service, great selection of games, and plenty of online tournaments.
2. Full Tilt- I like it mostly for the razz and low buy in MTT's and the bonuses are nice too.
3. Party- Very loose games, great selection of games, customer service isn't as good as the others, and I hate the interface.
4 Pacific- Never much cared for it, played it a few times, and decided I didn't like it very well.
Haven't tried many of the other sites.