Assuming I'm the only one with powers then the ability to control my bodies cellular development. If, however the world is filled with powers then I would probably want the ability to nullify all other abilities around me for 5 miles in every direction.
1-Rubber-Man. Reason: Never really feel pain and with a little training, I could kick ANYONES ass.
2-Super Speed. Reason: what looks liek one punch or kick could turn out to be 50 or 100. Insta-pwn'd.
3-Symbyote[Marvel Universe]. Reason: Look up "tenticle porn". I kid, I kid. If I could harness it's power, It'd be so fucking cool.
but if I HAD to pick one, it'd be the rubber power.
I want the ability to self-heal with an amazing auto-immunity to any new disease/infection. It's so nifty considering how much healthcare costs have risen lately all over the globe. I just want to grow old healthy :P