My original MM Ranking:
I think 3 is overrated. A long time ago I thought it was as good or better than 2 but while the core concept is great (more levels, more moves, Rush features), the actual level and boss design isn't as good and subjects you to way more BS/unfairness. 1 and 3 were both really bad for that.
I also didn't much care for 7 or 8. the art style, speed, gameplay, and feel of those games just felt off. Not bad games - none of them are - but wow I just didn't feel right playing them.
the first 6 were all immensely solid. 9 was outstanding, 10 was pretty good, too. 11 was also outstanding and the only non-8-bit game in the series I loved. I'm actually kinda surprised that, as of the time of this writing, not a single person has voted for 9, but there is a vote for 10.
For me a thing that puts MM3 in the bottom of the list, of the MM games I’ve played, is that you have to play each level twice (if I remember correctly) or something simillar. I’m usually not that fond of games that artificially lenghten the game in that way and no it’s not the same as backtracking in open world games like original Metroid.
I much prefer MM1 over MM3.
In Mega Man 3, you replay 4 of the 8 robot master levels. The levels are remixed quite a bit, and each has you fighting the robot masters from Mega Man 2, in different bodies. Actually one of the things I like most about the game. I wouldn't really call it artificially lengthening. I'm sure it was done that way to cut costs a bit, but it's still offering a different challenge. But to each his own.
As for 7 and 8, I think they're definitely worse than 4-6, but at the same time they offer a different feel. It's very rare that I'll actually replay 4-6, or 1 really, because I'd just rather play 2,3,9, or 10. But sometimes I'm in the mood to play 7 or 8.
Also, I keep remembering there are more Mega Man games. Mega Man and Bass should be included. There's an argument for Mega Man V on Gameboy too being included as it's a good standalone title.
So... my revised rankings would be...
Mega Man 2, Mega Man Powered Up, Mega Man 9, Mega Man 3, Mega Man and Bass, Mega Man 10, Mega Man 1, Mega Man 7, Mega Man 8, Mega Man 4, Mega Man 5, Mega Man 6.