Haki. By realizing your full potential and touching the spiritual world around you, you can gain awesome powers that include sensing things before they happen, super-armor, and the ability to knock out every weak-minded person in sight with pure awesomeness.
If you're on this wiki, you probably don't do too much with your life so awakening Haki would be hard, but say you were granted one of the three Haki abilities. Which one would it be? Yes, in One Piece you can have all three, but for the sake of discussion choose one ability that you would like to have the most.