Well I've been at Stars for ages now, and it's great. Lots of players, large variety of limits, great tourneys (from $1 to $200 buy-in) and SnG's, cashouts to my
neteller have always been almost instant, and on the couple of occasions I've needed to contact their support they've been quick and helpful. The players are better than at most other sites, but there's still a load of fish at the lower limits to be exploited (was sitting with a guy who had a 91% voluntary put $ in pot % at $0.50/1 NL a couple of nights ago, that was fun
).
That said I haven't had much experience with other sites, so I don't know the bigger picture. Tried Interpoker and Paradise and didn't like, tried UB which I like but I prefer the Stars interface (that and my UB connection has been really horrid over the last few weeks), so I only have money there.
http://www.pokerlistings.com/poker-top-lists <-- check here, they have some nice categorised reviews which will help you choose depending on exactly what you're looking for. Your best bet is to pick a few places and mess about with play money for a while before making a deposit at the one you decide you like the most.
