Wow, you people have great taste in games, was really happy to see Defense Grid and Space Pirates and Zombies in this thread.
As far as time spent, I have 408 hours in Dungeon Defenders, although I don't play it any more. I really like Castle Crashers for sitting back on the couch and playing with a controller. I think my favorite overall as far as constantly revisiting is Terraria. That big update was a heck of a treat as well. I also like Pixel Piracy, which was made by the same folks, but it just doesn't have quite the same replay value as Terraria for me.