I think I'd wish for something altruistic, not because I'm totally altruistic, but because I don't think I could come up with one single wish that would make me happy and not regret picking something else.
So... I'd wish for a cure to all diseases. My job here is done.
I think I'd go with what Scarf said. I'd end disease. It's both altruistic and also personal for me, given then disease is one of the things I'm scared most of in this world and the idea that I could abolish it from the Earth is just a very pleasant calming thought.
If I could wish for one thing, I would have to say equality. This could be on multiple levels. I think that if all people could live middle-class lives, with no one race, sexuality, gender, religion, or other beliefs thought to be more important than one another, many things like war and crime could be averted.
However, this sort of world would almost seem communist in nature, and in the past it usually ended up creating more problems than it fixed. So I might have to agree with Scarf in that I'd wish for a cure for all disease, as that would at least fix some of the world's problems.