what would it be?

The Keyblade.  Do you know how many bank vaults I can open with that thing!?  I'd be rich!  Rich! Lol.

To go back to a checkpoint and try again

Keep my hair in perfect condition, no hair gel, and dry instantly after getting out of the water.

Reality bending.

There are so many powers I'd want (Flying, shapeshifting, singing, magical swords and weapons....etc)

but then If I actually had those powers its very much likely that our society would classify me as a monster

 

I guess one power that would benefit everyone around me including myself would be the ability to heal/regen/cure diseases and ailments in a very short amount of time without side effects. That way I'd be able to help others as well as myself.

I would want the power to be a Keyblade Master, I'd be able to defend myself, or fight for good, I'd be able to get into anywhere involving locks, I'd be able to use magic, heal myself and others, create fire, ice, thunder, etc... I'd be set

Loading a save after death.

Manipulating time would probably be my favorite ability of them all.  I think it'd be awesome to travel through time, either to the past or present, or speed, slow, or skip time.  If not time, I would love to have the power of healing.

Albedo's ability to regenerate.

Cheat Codes

Okami's paint brush skills