That's really a difficult question, since there are so many diseases to be cured.
The one I would work on first would be Cystic Fibrosis. It is near , & dear to my heart because many years ago my sister was born with CF! Back in the 50's, it was rarely heard of, & if your child lived 2 years it was a miracle! At the age of 12, I saw many things that were terrifying, & very sad. My sister died at 22 months, & my family grieved for the longest time. They felt feelings of guilt, anger, & despair! Eventually they turned those feelings into being advocates for a cure, & with about 6 other couples, founded the Cystic Fibrosis Foundation! This is why I would start with CF!