Killer Croc in Batman Arkham Asylum.
Seriously, the way they hyped up this guy and made him out to be the most deadly creature in the asylum literally had me on the edge of my seat every time he showed up and I felt a crushing sense of doom and excitement when I heard Batman would have to enter his domain.
What followed was a boring walk around a sewer, throwing batarangs and Croc ever five minutes or so seriously wondering how he ever gained this kind of notoriety in the first place.
Seriously, they made something this awesome:
... and then wasted it on a crappy boss fight. I hope he's in the sequel because otherwise its a waste of a good villain.