3. Mega Man 2
Fun Fact: Capcom only allowed the Mega Man 2 team to develop the game while they worked on other projects, owing to the first game's lack of success.
Mega Man didn't sell very well, but Capcom decided to allow the Rockman development team to create a follow-up anyway. Smart choice: Mega Man 2 is the most popular and best-selling game in the series, and is also often considered to be one of the best video games of all time.
Mega Man 2 was a platformer like the first game, and featured vast improvements to the graphics, audio, and game mechanics. Players could recharge Mega Man's health at any time by picking up energy tanks which could be stored and used later, and a password system allowed players to virtually save their progress.
Players fought against eight Robot Masters instead of six this time around. The level design was better, the variety of enemies was better, and Mega Man 2's North American release also included an easy setting. That may have had a tremendous amount to do with how popular this sequel was, as it made the game so much more accessible to the general audience.