As far as OU, Infernape probably gets my vote. Pretty unpredictable at what he can really do. Pretty decent stats, and STAB Close Combat on any set just... hurts. I'd vote Choice Scarf Garchomp but... IMO, it's kind of a gimmick. Cresselia, Hippowdon, Skarmory, etc. kind of just... laugh at him on Outrage.
For a mention of underused and still quite threatening would be Rhyperior. I was amazed the other day when my Rhyperior (EVs: 4 HP/252 Atk/252 Spd Adamant) stood up to a +3 DD Dragonite that used Outrage. I'm still lost on what set to use for Rhyperior, but when I figure it out one day, there will be pain for those little Dragonites... hehe...