Mega Man II and III are my favorite NES Mega Man games; they probably could've stopped after that.
The very first Mega Man game was just kind of bland, the stages and bosses were, daresay I, uninspired. Don't know how well it sold, and if it didn't, how they got green lit for the sequel, but I'm glad they did. I've always been under the impression that the first Mega Man had a limited print of cartridges, considering the trouble we had to go through for the cartridge release back in the day, and we didn't even get the manual or retail packaging with it; just the cartridge in a box full of packing peanuts.
Mega Man V, I remember the boss weapons not even being useful, much less useful on the other bosses. I blazed through that whole game just with the Mega Buster - not that that's really something to brag about. Capcom made the game a lot easier than previous series installments.
Mega Man X, X2, and X3, I'd say they could've stopped after that, but X4, I actually liked, because you could play Zero as a main character.
In X3, Zero was just kind of a side app, and you couldn't fight bosses as him. I think you could only summon him once per stage, too.
Mega Man X5 was absolute dreck, given the poor translation, forgettable music, stupid-as-hell plot, and absolutely ridiculous boss names, that contradicted each other between what was in the instruction manual, and what was in the English translation of the game.