Mega Man: Dr Wily's Revenge

When you defeat them, you can take their ability as your own, which can provide an advantage against other types of evil robots. In this Game Boy version, Mega Man must fight against Ice Man, Electric Man, Cut Man, and Fire Man. After he defeats those four, he battles Quick Man, Heat Man, Bubble Man, and Flash Man.

And after defeating all eight Robot Masters, there's Dr. Wily's Space Node. However, it won't be an easy challenge without using passwords to save your progress past certain milestones.