The Mega Man (MM) series was one of the best on the 8-bit consoles, though it had a slow start by the second & third games there was a huge fan base.
Quite right too, each game was an excellent evolution, bringing with it new characters, moves & better game mechanics. MM3 tried to better every aspect of the game & it succeeded.
Capcom actually invited the public to help design the bad guys & by the 3rd game, they received around 50000 submissions!
Years later the games lead artists Keiji Inafune, said he regretted much of the developments brought with MM3. While MM2 was great & people will argue which was better, both are amazing games.
The series became synonymous with greatness, none of the later games beat the excellence of the second & third game in the series.