...What would it be?
To keep this discussion sensible and interesting, two rules:
#1. Only ONE superpower allowed. "I want Superman's power" doesn't count, because he has too many powers: Super strength, super speed, the ability to fly, invulnerability to harm, heat vision, X-ray vision, freeze breath, and super sense of hearing all count as ONE power each. Similarly, Wolverine has three powers: indestructible skeleton, retractable claws, and regenerative healing factor. Same goes for several other X-Men and Spider-Man, and many other super-heroes.
#2. Certain powers are too cheap to count, because they are too powerful. These include, but are not limited to: Molecular manipulation, Probability manipulation, Reality warping, Reactive evolution, Omniscience, Cosmic awareness, or Magic. Immortality and Invulnerability are acceptable, but they must be separate powers. Telepathy and Telekinesis or Precognition and Telekinesis must also be separate, although Telepathy and Precognition can go together. Also, certain kinds of molecular manipulation are acceptable, if it's narrow in scope (ex: the ability to manipulate metals or plants). Green Lantern's near limitless power over all things not yellow
isn't narrow enough to be acceptable.