^^ I feel like every group I'm a part of asks this question at one point, and my answer has changed each time.
I once said healing, because helping others is a good and beautiful thing.
I once said invincibility, because then nothing could hurt me. I once said invisibility, because I didn't want to be seen.
I once said flight, because then I'd be free. I once said time control, because then I could fix all my mistakes or even just put the world on pause when I need to rest, seclude, or just try to catch up.
I once said teleportation, so that I could go wherever I wanted, whenever I wanted. I also once said immortality, but on this mortal plane, that would be a curse.
I once said wisdom (like Solomon), because then I'd always know what to do. I once said psychic or wizardry power (such as matter or elemental control), because I wanted subjugation of something due to a hidden agenda of wanting to prove something of myself.
Do you know what I realized though, after all that? None of those things would truly be good or make me happy. In fact, they'd integrally lead to my (or other's) destruction. So now, it's a toss up between two which I always seem to come back to...
The power to give others powers, or the power to take theirs away.