So many to pick from. I will pick one of the following. Although I think I would use them mostly to mess around to have fun than helping/hindering humanity..
- Invisibility, so I could steal stuff, pull so many fun pranks, and steal more stuff.
- Teleportation, that way I can visit any of my friends around the US/Canada, and go sightseeing around the world.
- Telekinesis, because it would allow me to lift stuff/be lazy, read people minds or let use elements of the earth so I could set squirrels on fire. Or bears/cougars if they tried to attack me.