I think just the ability to not need sleep & not get tired. It sounds pretty basic at the start but I would have so much more time to do what I wanted to and enjoy life and work harder to get places.
If you can have any superpower - which one would you want to have?
For me, my top 2 would be:
Teleportation - to be able to travel anywhere in the world in an instant.
Healing - to be able to heal myself and others with just the touch of my hand.