Hmmmm....
Well, League of Legends. I guess. I only play it because its the only game my brother has been playing as of late, although his need to be ridiculously competitive at anything sucks the fun out of it.
I like Warframe because, since its Co-op rather than Player vs. Player, which solves the Pay to Win problem a lot of F2P games have.
War Thunder, because dog fighting is awesome. And of course, Team Fortress 2.
That's about it. I guess I could come up with more if I tried. I mean, Soldier Front 2 is a F2P game that I only play for the Fun House game type, but I wouldn't call it my favorite.