PokerStars, PokerStars, PokerStars. No others for years; the Black Friday domain seizures scuppered other half-decent sites and others left the Crypto-Logic Network.
I play on several poker sites, including Pokerstar, 888 and GG poker are my main ones, it's what makes me win more, but not every day, of course, right?
The most tournaments I have played is at pokerstars as I find they have the best selection of stakes and games with the most players.
I think GG is also a good site who get many players as I like to play large field tournaments where I can multiply my buy BIG if I make the final table.