It depends really...what do you value?

Lots of games and with potentially the same number of weaker players, but horrible customer service, Party Poker.com would be a good location.

Large site with skilled players, but great customer service and large variety of tournaments for every skill level and bankroll, Pokerstars would be next.

A medium sized network (many skins), but horrible rake structure, the Poker Network (Poker Room, Eurobet, Hollywood Poker, and so on) could be your another possibility.

I've played at these three...take your pick.

I haven't played at Prima, Crytologic, iPoker, Absolute, or Ultimate Bet, I'll let others elaborate for those sites.