Cancer, since world hunger is a man-made problem, that could theoretically be solved whenever humans collectively decide to... well, actually give a shit.

Tough choice. On one hand, hunger is something that only exist because of the selfishness of human beings, so curing hunger would rob our species of the opportunity to evolve to be more compassionate and generous, while cancer is a more incidental issue that arose and caught us off-guard. On the other hand, cancer is, IMO, the result of excessive technology, artificial chemicals, food additives, pollution, etc., so ultimately cancer is human-created just as much as hunger is, and the same mentality that's causing cancer is also destroying the environment, giving us 24/7 stress, and disconnecting our spirits from nature.

hunger, because it is a fundamental need that everyone should have met