The first Mega Man sold modestly. Capcom gave the team permission to make a sequel on the condition they do it around their other assignments, which is to say on their own time. Mega Man 2 (1988 in Japan, 1989 here) is the one that turned a cult game into an institution.

The loop is pure design candy. Pick any of the eight Robot Masters in any order, beat one, and the weapon you take off him shreds another. It's rock-paper-scissors wrapped around razor-sharp platforming, and the order you work out yourself the first time through is a thing you remember.

Capcom sourced the bosses from a public call for submissions and got somewhere north of eight thousand entries, which is why the roster is such a grab bag: a man made of wood, a man made of bubbles, a man who is mostly a flamethrower. Metal Blade is the famous outlier. It fires in eight directions, costs almost nothing per shot, and kills most things in the game including its own user's difficulty curve. Nobody has ever agreed on whether that's a flaw.

The quality-of-life additions matter as much as the weapons. Energy Tanks arrive here, so a boss fight no longer depends on whether you reached the door with health intact, and a password system means you can stop playing and come back. Mega Man 1 had neither. The US release also added a Normal and Difficult toggle, with our Normal sitting below the Japanese standard.

And the music. Takashi Tateishi's Wily Stage 1 theme is one of the most-covered pieces of chiptune ever written, and Air Man and Bubble Man are not far behind.

Hard, fair, and endlessly replayable. This is the NES action game I come back to most.