Certainly cancer, for pure selfish reasons. I'll probably get it more than once.
It affects many people, and unlike many of the other options, there is no prevention (for many types, although there are things which will lower risk). It affects young and old, while many of the options are more biased towards the elderly who have lived relatively full and rewarding lives (sure, broad generalization, but compare it to a teenage leukemia patient, as an extremish example). Next of those choices, by the same line of reasoning, would be MS, Lou Gehrig's, etc.