In order of favorite to least favorite: Full Tilt, PokerStars
, Ultimate Bet, Carbon, Absolute, and then Bodog.
I like FullTilt the most because that's where I've been able to establish the biggest bankroll. That site, in my opinion, offers the best freerolls. With a max of 2700 entrants, there's always a possibility of making the money. That's how I've established a bankroll, through the sites great freerolls, which run quite often through the day.
It's also rather easy to make the money in PokerStars freerolls, too. The only difference is that it takes two tournaments to make it to the money.
FullTilt and PokerStars are my two favorites; the rest don't even come close.