20. Mega Man 3
Fun Fact: Proto Man was originally called “Blues” in the Japanese version, but Capcom North America changed the name because they didn't think “Blues” made any sense.
Mega Man 3 introduced a new character, a robot dog named Rush, who could give Mega Man the ability to jump higher, fly around the screen, or travel underwater. Another new character named Break Man appears as a recurring mini boss, and at the end of the game is revealed to actually be Proto Man, Mega Man's brother.
The basic structure of the series remained unchanged, although additional stages were set between the areas for the eight Robot Masters that Mega Man had to destroy, and the final area of Dr. Wily's fortress. Mega Man 3 also introduced the slide maneuver that allowed Mega Man to avoid bullets and slip under low walls within the levels.
Mega Man 3 was considered by some critics to be the best game in the series, while others took issue with the increased level of difficulty and visual flaws.