Depends on what you like. On pokerstars there is always find a tourny that fits your bill. PS has way more tournys then FT will ever have but be aware that PS does not offer rakeback. FT is recognized as a site where the high stakes cash players go. Sites normaly offer 27% rakeback for this site.
My personal opinion:
#1 Poker Stars
#2 Full Tilt
My personal opinion:
#1 Poker Stars
#2 Full Tilt