Ugh, couldn't we limit this to one? Once you get two or more powers, you can just stack 'em all and make yourself Superman without the overt smugness.
If I had to pick ONE power out of EVERY power conceivable (notice how the limitation of one forces tough decisions), I'd pick super speed. It's a versatile ability. Sure, you might be weak yourself, but inertia is very powerful. Vibration is powerful. Not to mention, speed would be a useful power for HELPING PEOPLE. A lot of folks go with the old destructive super powers, but what use is shooting eye beams if you don't want to kill anyone? And none of that "oh, it's like Cyclops, he just knocks people out" nonsense. I don't know what Stan Lee was smoking when he drew somebody LASERING A GUY IN THE FACE and decided that wasn't considered a death blow.
Anyway, with super speed you could take pictures of a crime scene as the crime was happening, snatch all the guns out of the robbers hands (wearing gloves so as to get the prints), remove any masks they might be wearing, turn on the silent alarm, make sure the cameras in the bank are working and then handcuff the robbers to a table. Compare that with super strength, where you punch a guy through a wall and get a life sentence for murder, or invisibility, where your attempt to stop a bank robbery becomes a near-death experience because a fart gave your position away.
Edit: I saw somebody mention ZA WARUDO. I have to say, that'd be probably second in the abilities I'd want. Again, though, time manipulation and super speed could be said to be the same ability. As you speed up, time slows down, etc. It could be said that Za Warudo is not so much an ability that stops time as one that accelerates one's speed to beyond superhuman levels. That would make sense considering the nature of Jotaro's Stand and how it tends to work for him.