For athletic purposes: Michael Jordan in his prime. It'd be fun to find out what it feels like to be the best basketball player ever, to be able to dominate anyone on the planet. Secretariat would be a good one, too...just to feel that superior.
For "other" reasons: David Beckham. Probably the most recognizable, wanted man on Earth. Winkyduck said Derek Jeter, but that's limited to just American women...Beckham could score with just about any bitch on the globe at the drop of a hat. Lucky bastard.