I think the fastest way is to complete all missions, side missions, takeovers and challenges, then collect resource items and collectibles.
I had a chance to kill the best, but then I remebered suicide is a sin.
Red Dead Redemption (all DLC except free coop stuff)
Splinter Cell: Blacklist (Multiplayer)
Borderlands 2 (no DLC)
Ghost Recon: Future Soldier (no DLC)
Assassins Creed: Revelations (no DLC)
Contact me if you want to boost any of the above games.