Lycanthropy for me. Enhanced strength and speed, augmented senses, incredible damage resistance and regeneration, teeth and claws, and a terrifying appearance to boot. I wouldn't need a costume, and I wouldn't have to run off and prepare for a battle. "Something's wrong? *transform* I'll be right there."
I'd be one of the more properly-proportioned werefolk, mind you. Not like the Van Helsing Big-ass hulk werewolves.
Alternatively, I'd be a nigh-indestructible dark-magic powered snowman. I would teleport with a whirl of fresh snow, curse people by wounding them with my wooden claws, and freeze the everything around me with a blast of chilled wind. If I melt (which is very hard to do), I just wait until someone else finds my hat, which is nigh-indestructible and the source of my awful power.
Or, failing those two, just give me overkill jedi abilities. Nothing says owned like throwing ten lightning-charged cars at an enemy.