3. Mega Man 3 (1990)
Expanding on the form and function of its preceding entry, Mega Man 3 is one of the most challenging and rewarding gaming adventures you can have on the NES. Filled with awesome enemies (Snake Man could very well be my number one robot master), level design that is inventive and themed well for each robot master, and feverishly fun music, this is a hallmark of the entire series. But, it's most important contribution comes in its implementation of a canon-defining story. This is the game that presents us with Break Man, a mysterious red robot that pops up from time to time to battle with Mega Man. However, after the end boss fight, Break Man saves an unconscious Mega Man from certain destruction and drops him off with Dr. Light. It's then that Dr. Light reveals Break Man to be Proto-Man, his first creation and Mega-Man's brother. This ending shapes the Mega Man world into just that: a fully realized world with characters and emotions that may not be as prevalent in the preceding games. If just for that reason, Mega Man 3 is one of the most crucial entries in the franchise.