Mega Man 6 by far. The others are all great games, but 6 really is not. Sure some graphics are nice, but the levels aren't that interesting. Also the robot masters are just awful, the weakest collection of bosses in the series for sure. They all just shoot stuff at you... there's none of the variety of boss types that you find in most of the other NES games. The new Rush Suit stuff doesn't work that well either, this is classic Mega Man, not Mega Man X, and that feels like something from an X game sort of. Also, Mega Man IV for the Game Boy came out before this, and Mega Man V was in development or out (depending on region I believe), so why doesn't this game have the money and shop systems from those games? All I can think of is because the game was obviously a cheap final effort done in the system's last days just because of how successful the older games had been. That had been true for some time though I think, and through the fifth game the NES games were great (indeed Mega Man 4 is my favorite one of the NES games), but the sixth one's just not even close.
Mega Man 1 might have aged pretty badly, but it's such an amazing game for its time and still such a classic that I can't rate it badly... and plus, it is still great despite its flaws, particularly in versions like the Mega Man Anniversary Collection version where you can actually save. 2 almost everyone loves. 3 isn't one of my favorites, but it is still a good game... I might rank it fourth or fifth in the series, but there's a big gap between that and MM6. 4 is my favorite as I said. It's just an amazing game... the charge shot was a fantastic addition to the series, and the game has my favorite bosses and music too. 5 is pretty good; it's too easy, but other than that it's great, with some interesting levels, cool robot masters, and good new ideas. 6 isn't a bad game or anything, it is still a good, fun game, but it's way behind the first five for sure.
My order, best to worst: 4, 2, 1, 5, 3, 6 ... though 1, 5, and 3 are pretty close and I could see putting those in another order. I could easily swap 5 and 1, it's hard to decide between those two... but also, maybe I rank 2 a bit high, I mean it is great but I don't love it as much as some people. Whatever the case though, as I said the first five are all fantastic games, which definitely makes it harder to choose between them. Being just good the 6th game's definitely in last, but still it is a good game.
