Rather than do the usual "what hero would you be", I'm going to run with "what sidekick would you be", because let's face it, choosing a hero is a piece of cake. To eliminate all the "Duh, Robin, I might get to drive the batmobile" answers your choices are as follows:
Patroclus: Sidekick to the greatest hero of them all, Achilles. Con: Did absolutely nothing whatsoever of note and not even Homer could make his end of the friendship stand up to scrutiny. A choice entirely bound in reflected glory.
Danny Glover, Lethal Weapon: You get some good lines and shoot a few people. Con: Your partner really is that crazy and if you were dragging out that "2 days to retirement" bit any futher you'd be Larry King. You're also stuck in the 80's which is a fucking horrible desolate wasteland of a place.
The narrator, Fight Club: You do a whole load of monumental stuff. Con: You're not even the biggest draw inside your own head and your fans are going to castrate you.
Ron Weasley: You get to shag Hermione. Con: Your entire life exists in the shadow of another man and nothing you ever do will be good enough. Hermione's a bit of a ladyboy when she gets her hair cut after you all finish *****.
James May: You get employed to potter about being nice about things you're interested in. Con: You're no Clarkson or Hammond and nobody's quite sure what you're doing there.
Harpo or Zeppo: You'll make a fortune. Con: Groucho could easily replace you with cardboard cutouts.