Wizard101 for a long time. But I stopped because it was so limited (movement) and it was pay to play, basically.
Minecraft, long time favorite game. I still like it, but it's really boring now so I don't play it as much.
Left 4 Dead 2, 300 hours on that game; not that much, but whatever. I'm bored with that game now. Zombies used to be my thing
Currently
osu!. There are so many elements in this game that ensure that you never get bored of it!!!
And I'm starting to kinda get into
Team Fortress 2. It's okay, it isn't as exciting as I anticipated it to be....
So yeah, I don't have a favorite all time game