Technically, both the ability to run incredibly fast and jump incredibly high would have their roots in having super enhanced leg muscles, so I vote to have awesome legs, thereby having both super speed and super jumps.
Though if I could only pick one...
...it would be flying in a heartbeat. There's a girl I was close with in highschool and I promised her she'd be the first one I ever took flying if it were to happen by some freak accident that I got that ability...