Black & White 2: greatest potential for horrendous abuse of power. The things you can do in that game would make Uncle Ben return from the grave just to have a heart attack and die again.
Speaking of Uncle Ben... Spiderman 2 (the movie tie-in) has the best line-swinging ever. They absolutely nailed it in that game.
Portal 2's coop mode has the best random betrayal of trust - and not just the ability, but the encouragement to do so. Nothing says friendship like dropping your partner in miscellaneous, lethal goo in the middle of an important task.