Nominate The Challenge: The Void for Best Social Game
The challenge design in this game was just incredibly good. A vast majority of new ideas and a vast majority of those worked really well in the format and were more fun than most other social game challenges I've played. I'd love to see more games going forward take a page from this game in challenge design, not that that's easy as the mods obviously did a ton of work on that.
The social element was worked in such a way that it stayed fun and light -- you could do as much or as little of it as you wanted and still have a shot to do well, which I thought was pretty neat. As far as I know, the Void/Black Hole mechanic was original, and it worked well to create what the game was trying to be.