I'll also say Fallout 3's pre-DLC ending, and Fable 2's. But doubly so because they set both up to be these huge, climactic, exciting battles that would put all your skills and knowledge to the test, even with all your super-powered equipment, and then,
Liberty Prime (though AWESOME) does all the hard work for you, and doesn't heel-face turn at the end; and Lucien goes down with one shot, even to the HAND.
WHY?! He was absorbing all your friends' abilities! THAT BATTLE SHOULD'VE BEEN AWESOME!
WHY?! He was absorbing all your friends' abilities! THAT BATTLE SHOULD'VE BEEN AWESOME!