Easy - I'd be an extremely powerful mage in a Dungeon & Dragons adventure, slaying goblins, orcs, giants and dragons alongside my trusty adventurers. Besides, I always dreamt about inspiring awe in a medieval pub by forcing the flame from the fireplace or candles to dance at my will, and silence a drunken bully using a single harsh stare like Raistlin of the Dragonlance saga.
OR, if I'll would grow tired of that (and I doubt that I will
), I'll be Lord Vader, a starfleet captain, an extremely skilled space fighter pilot in a huge dogfight with dozens of ships, or even a naval commander in the 18th centuri.
Edit: oops, failed to notice that I can only pick one. OK, I'll go with the first one, then 