I'd want power over gravity, but not the lame "Crush you with super-gravity" or "Pull a meteor at the Earth" control over gravity that's usually portrayed in fiction. I'd have the power to affect the amount of force exerted by an object's gravitation onto other specific objects. So while I could technically do that, I could do several much more devious things, like "The Earth's and Sun's gravities no longer affects you" *cue the person suddenly flying off into space as he keeps travelling in a straight line while the Earth is moving*. Depending on how loose an interpretation is allowed, one could become the god-king of space by using the gravitation of random particles floating through the air to affect any object's motion however one wanted to. Of course, for this power to be of any use I would also need super-intelligence to be able to comprehend the complicated systems I would be manipulating.

As for heroics vs. villainy, well, they say "Power corrupts, and absolute power corrupts absolutely". I think I would start out with noble intentions (though I would use my powers to solve social problems rather than stop crime) but I would quickly fall to using my power simply for personal benefit. I don't think I'd go into full villainy mode of oppressing others' rights, I would definitely develop a "me first" attitude and probably wouldn't mind hurting a few to get ahead.