For me its a toss up between time control or shape shifting.
Time control would be epic fun and thats complete control of it, being able to effectivly time travel to the past or future, being able to slow, stop or speed up the passage of time for myself or anyone or i choose, or being able to reverse them.
Shape shifting as a super power is completely over powered if you were able to take any shape or form you could beat almost anything... except maybe another shape shifter, and it doesnt matter if you get hurt or anything cus you could just shape shift it better.