In FPS games: I'm usually the sniper or the grenadier (depending on if a game has decent grenades or not).
In RPGs: Thieves, Assassins, rogues... anything with high stealth and stealing ability. While barbarians and fighters seem like fun, they get old, fast. Magic users just take too much time for me to want to deal with (excluding guys like a warlock).
In Fighting Games: The fast guy that you can hit out in a single punch, but will have a fucking hard time doing it.
In my Social Circle of Friends: I'm essentially the leader. I swear, whenever I'm going to be late or unable to show for a while, I leave the plans in the hands of people, who I assume are capable of handling it. Once I am ready to join in, I discover that everyone has bailed because no one stepped up and said, "Hey, I have an idea of what to do." I suppose "babysitter" is another good way of describing me when it comes to hanging out with a bunch of other 20-something manchildren.
At Work: That guy... you know... that guy who knows how to do everything and can do it quickly, but is generally lazy about it because he knows that it won't take him too long to get anything done. Yeah...