Either Halo 3 or Borderlands 2.
For a while there, I was the number 1 player of Borderlands 2 on Raptr. I can't be sure if it's still like that, as I've gone to play my 9 other yet-to-be-started games for a bit, but yeah. It was a lot.
Halo 3, I have no clue, as I wasn't on Raptr back...