It's like comparing 200 diseases with 1... I'd chose cancer. While developing countries don't have access to meds for HIV they also don't have them for any type of cancer or basically any "big" disease at all. Some forms of cancer are very much treatable but some of them are just too destructive (pancreatic, lungs...).
While I understand that babies in developing countries can't chose whether they'll have HIV or not most cases of HIV could be prevented. Nobody choses to have cancer. It's also not preventable in the same way as HIV.
By the way, I was reading the other day about a portuguese guy who found a protein, a single protein that when changed could kill cancer cells in about 9 hours
here's the video: