I have several thoughts. Thinking out loud here...
1) Superman. All-Star style Superman, that was my favorite depiction, and really highlighted the Fortress of Solitude and the incredible resources at his calling.
Pros: Superman. Duh.
Cons: Overwhelming responsibility, an enormous array of massively powerful beings trying to kill me.
2) Jamie Madrox, the Multiple Man. His powers absolutely fascinate me, I think I would love that setup, and no real arch enemies to speak of.
Pros: The ability to try everything, to learn everything. To never truly be alone.
Cons: Psychologically, it would probably be devastating to acquire now without growing up learning how to handle it. Also, that whole mutant crap thing.
3) Ambush Bug. Seriously. He's not insane, he just sees things ... differently.
Pros: Global teleporter. No real enemies. Sees beyond the fourth wall. Very funny.
Cons: Ok, yeah, he's insane. Suddenly seeing through the fourth wall would probably do that to me, too.
4) G'nort. Again, seriously. He's got a Green Lantern ring, and almost none of the responsibility. What's not to love?
Pros: Great friends in the JLI. His arch enemy is the Scarlet Skier. And did I mention the power ring?
Cons: He's a dog. Nobody takes him seriously, always cast aside.
Tough choice.
Hmm, Hercules seems pretty happy...