Yeah the magic cancer bullet would probably be on the top of my radar- however this is probably unrealistic as this would ever occur. Even finding a cure for just one type of cancer is almost farfetched. This cure would also have to have no issues with resistance, as in, once you have used it the cancer never comes back. Actually now that I think about it, coming up with an effective cancer vaccine would equally just be as good. You would get it when you are young and would not have to worry about cancer for the rest of your life.
If I came up with any of the above it would be good for 2 reasons- firstly and most importantly, it would help hundreds of thousands of peoples lives. Secondly- imagine how much respect, recognition and money you would have for doing this. If I did have heaps of money like that though, I would set up a foundation that gives money and grants to research groups tackling a variety of societies medical issues. I think I would celebrate with a mad huge party, something like a mini festival of Djs and bands for everyone that helped contirbute along the way/ my friends (since I would have the money to do so). One of the first people I would tell would probably be the intellectual property people to get the cure patented- lol.