Mega Man 3 — Proto Man, Rush, and the Slide
The sequel that added a dog, a rival, and one essential move.
Mega Man 3 (1990) took the perfect formula of MM2 and bolted on the pieces fans never let go of: the slide (suddenly you could dodge), Rush the robot dog (your transforming ride), and the mysterious whistling rival Proto Man.
It's bigger and more generous than its predecessor: eight Robot Masters, then a gauntlet of remixed Doc Robot stages, then Wily. Some say the rushed development shows; most of us were too busy sliding under everything to notice.
That slide changed the series forever. Every Mega Man after this one assumes you have it.