My favourite poker room online is PokerStars. Reason is simple - I've only tried two and they merged. I am a small time player and have no interest or need look elsewhere.
The question did not mention online vs live. So, my favourite live poker room is The Arena Poker Room at Talking Stick Resort, in Scottsdale, Arizona. The state does not allow no-limit poker, but there are spread limit games that are just about as no-limit as you can get. There are about 45+ tables with a variety of games and stakes.