Well, he has four different types of eye beams, two different types of super breath, super intelligence, super speed, super strength, two different types of invincibility, flight, I'm forgetting nine or ten more... Pre-crisis he could also change his physical shape and had various forms of minor telepathy. His super math power is canon, though for it's
one and only appearance he picked up an extra digit somewhere, so "super" doesn't always mean "correct."
Basically: for most of his history he was completely open-ended and writers could add any new power they needed for a story. Green Lantern and Batman had the same thing, always just happening to have something impossibly single-use on hand for situations they couldn't possibly foresee.
His biggest superpower, though? He's protected by iron clad contracts - for most crossover events, he's usually off limits entirely or not allowed to be beaten (sometimes even hurt) by non-DC characters. Crossovers have been cancelled when writers don't stick to that one close enough.
The interesting thing is, he's not even the most overpowered thing in the DC universe, since the Lanters can replicate most of his feats and add in the ability to create any object they can think of, or to simply banish an enemy from their dimension rather than fighting it fairly.