Mega Man 3

Just edges out Mega Man 2 as the best title for Mega Man on the NES. Less straightforward boss names where you can't always guess what works well on who... and I liked that. Also marks the arrival of Proto-Man.

A natural progression of 2, adding great staples to the series such as Rush and the slide. While the sub-weapons are kinda meh, the boss fights are all great despite the padding that comes with the doc robots. What makes the level design great is that on top of having a lot of neat gimmicks built upon from the last game, they also add a lot more sub-bosses while also finding inventive ways to make you use the slide mechanic. The Wily Castle in this game was oddly forgiving, though.

My favorite of Mega Man's NES era. The addition of the slide and rush parts gave us even more tools to experiment with, the levels were tough as nails, the graphics and music were gold, and the boss fights were nuts. The biggest flaw here is that the Protoman fights were repetitive.

A long one. The slide and Rush make their appearance, giving you some more flexibility in moving around on top of the usual growing flexibility in attack options (though I do wonder how many players actually found a compelling use for the submarine form).

Bullshit enemies, way worse than MM2 which had a reasonable level of challenge. This has un-dodgeable enemies who take off a third of your health in a single shot.

About as enjoyable as its masterpiece predecessor. Capcom just damn well knew how to make NES games.

The one where you can slide. The finale is a god damn gauntlet of hell (plug this into every NES Mega Man review)

Seemed less difficult than the previous, but otherwise is more of the same.

You can argue which is better: 2 or 3 but it is certain that both are great and the peak of this franchise.