Pokerstars is the biggest, and there's always a good tourney going -- or 15. Always plenty of cash tables at all levels. They have fun games like Badugi, 2-7 games, 8 game. Puts on poker series, with money guaranteed, and lots of guaranteed tourneys.
That said, I mostly play at Full Tilt, because I feel comfortable there and have lots of poker friends there. Also tons of tourneys, lots of action. Also has special poker series. Pros are highlighted and sometimes play at small stakes tables for fun, and in smaller stakes tourneys. Bust a pro, get a T-shirt. They also have the FT Poker Academy, with lessons, sessions and such to help you improve. Love that.
Bodog is fun, and has pretty soft freerolls
. Not nearly as many players, so it's hard to get some games going.
I like limit holdem SNGs, and am more likely to be able to play at Full Tilt -- and i know most of the regulars at my level and have notes on them.
I don't play at UB nor AP hardly ever anymore. I don't trust them.
I'm in the US, so I have fewer choices than those who are based elsewhere. I hear Party Poker
is still going...