Say for instance right now at this very second you can totally, completely cure one medical condition and wipe it from future existence. Which would you chose to cure?
For me, I would say nut (peanuts included) allergies. Mainly since I think some are very tasty and some people have very serious allergic reactions to them. Plus if people can't enjoy pistachos, walnuts, almonds, cashews, peanuts, etc. Well, that just plain sucks.
Figure a lot are going to say cancer, but IMHO there's a shit ton of research being done on that. I would kind f be surprised if we can't find a cure at some point.