I'd say the final confrontations in Fable 2.
To begin with the giant artifact that the bad guy is making is just a giant pile of hit points with some minion spawns, that takes forever to wear down but otherwise isn't all that special.
Then when you finally confront the main bad guy you either dispatch him with one button, or one of your companions does it for you. I suppose this was somewhat appropriate, but a little disappointing. The guy was mostly a mastermind rather than a powerhouse, but still I was sort of like "That's it?".