What's your favorite F2P game?

You know, I'd have to agree with the people that are saying Dota 2 because that's a solid game with a lot of variation. It's one of the best F2P games out there, and I know plenty of people that are basically addicted to it. I probably play about 10 hours per week, which I consider a reasonable amount.
 
For me personally, Path of Exile is my favorite F2P game. The whole game is relatively free to play practically, and is a worthy Diablo clone. The only thing paying adds to the experience is gear that you can easily transmog, portal colors, magical effects that don't change your characters strength, just the animations, and dancing animations as well. All paying does is add a sort of status to your character that says to the world "I paid, and I got cool shiney stuff to prove it."
 
TF2 for me, but I don't usually play F2P games. The few time's I have decided to play it it has always been balanced and real fun, especially playing as the spy.
 
I have heard about Dota 2 but I have never played it before. I don't usually play F2P games and so I may look at some and just maybe I will play. If they are on the Android platform then It will be easier to get me to play them than on a computer.