I play just about all the games on there. The more popular games would be Farmville, Mafia Wars, Vampire Wars, Dog Wars, and Castle Age. It just depends what your own personal preference is, to as which one is good.
I've heard a lot about Farmville on Facebook - although I've never actually played it. But everyone else seems to really like it. Otherwise I've also heard a lot about Mafia Wars on Facebook. Those are the only 2 really popular ones, I think!
