What game do you most enjoying achievment hunting for?
In my opinion, Starcraft II has some of the best achievments. They are numerous and varied, and the campaign achievments give you a good reason to go back and play the level again with and force you to alter your play style. I'm not an achievment hunter by nature, but Starcraft II is one of the few games I will go back to every once and a while for the sole task of nabbing a few more.
What are your favourites?