The choices of super powers really make it hard. They are all good super powers. Choosing between being able to speak and understand any language ,to make any two people agree on something or time travel. Just think of the possibilities of what you could do with any of these.
I think I would have to pick the ability to time travel. Not just because I have been a fan of Dr. Who way before he was cool. In fact I have been watching him from the beginning .Way before the special affects or the sonic screw driver.
I would pick time travel because of all the things I could go back and see for myself. To answer questions that no one has been able to answer and to see what really happened. The hardest part would be stopping myself from trying to change the really bad things that have happened in the past.That is the one thing that I would have to be really careful of.
While anyone would want to back and change the horrible parts of history. I think it would change everything to the point of having some very bad consequences.