I would say AIDS. because cancer is not one disease, but a name for a cluster of similarly symptomed diseases with a myriad of causes and/or potentials for cure. So you'd have to find a cure for each seperate variety. Tall order.
And it gets to me how so many MILLIONS have HIV and they are going to die of it as their govts are too corrupt or too poor to get the drugs we have access to over here. I'm assuming you're talking worldwide BTW. We're talking people who can't read or write, know nothing of disease or the mechanisms of its transmission, who see it as some kind of random plague and have no way of understanding how to prevent it - education would be a great help there I think. That and donating millions of condoms to be handed out free of charge. They're not expensive to manufacture.
There are 20 millions South Africans infected with HIV. Most can't afford any treatment whatsoever. THey are going to die in slow and painful ways...