Depends on how you define it...
If we're talking "Sky's the limit" stuff, I'd go with the ability to alter reality with my thoughts. ( Conscious Thoughts only because I've read Lathe of Heaven and seen Forbidden Planet.) Works like hyper intelligence, but MUCH faster! Gamer nerd dude today, gamer ruler of the world dude in 5 minutes. And #1 rock guitarist of all time. And ripped like a gym rat. And...
If you're talking classic stuff, Invulnerability (not Immortality) has mucho appeal, especially if your invulnerability allows you to negate certain kinetic effects, a la the Juggernought.
On the other hand, I LOOOOOOVE jewelry, so a Green Lantern ring has a certain appeal as well...