1. Pokerstars - By far the best tournaments around, and plenty of players.
2. Full Tilt Poker - Love their design, and they are one of the few to offer my best game (razz).
Play at Party a lot, also have Pacific, Full Tilt, Royal Vegas and Stars. End up at Party a lot, something about Full Tilt i don't quite like. Don't know if it's their software or perhaps just caught in a slow game, not sure - I'm a newbie there. The others i use but not a great deal. Party seems to have all the action i need in all facets, plus it was where i started and learnt so it's almost a habit now.