Since I'm a huge comic book fan, I've naturally thought about this a lot. First, there are a few powers I used to think would be awesome but would ultimately have negative consequences:
-Telepathy. Being able to read others' minds might seem convenient at first, but imagine every random weird thought you've had. Now imagine everyone having those kind of thoughts, but only you can "hear" them, and nobody would believe anything you said. Some guy thinking about killing himself, or someone else even? Nobody would take you seriously. Even worse, imagine being able to read a significant other's mind. Eesh.
-Telekinesis. I know i would use it so much that I would be come soo lazy, that my body would fall out of shape. Same with anyone else. You'd get fat, or unhealthy, and die sooner.
Shapeshifting and invisibility aren't as bad, but it would be hard for anyone who knew your powers to really trust you.
I myself would go with flight. Flight that takes energy, similar to running, but not so difficult as to be unable to reach pretty fast (aka not superman) speeds, and you could carry someone with you. Though controlling fire/ice/water/earth, any elemental power, would be a lot of fun too.