Mega Man 2
Capcom seemed to think that their original Mega Man game was good enough for a sequel, because sure enough, there was a sequel. Mega Man 2 is often considered the turning point in the Mega Man series. The graphics and sound in Mega Man 2 were so good, and so much of a step up from its predecessor, that the audio/visual aspects seem almost revolutionary.
The number of bosses in this Mega Man game took a step up from six to eight. Air Man, Bubble Man, Crash Man, Flash Man, Heat Man, Metal Man, Quick Man, and Wood Man form the ensemble as one of the greatest collections of bosses the Mega Man series has ever seen. Even today, many people consider Mega Man 2 one of the greatest MM games ever released.