And I make the counter argument, that it's very fundamental gameplay makes it not the best. because they had to remove two staple moves from the main character to make the game challenging. That in and of itself is artificial difficulty. Much like 9 had artificial difficulty by throwing [parasitic bomb] at you from offscreen. The extra characters were nice and all, (Bass sprite still felt lazy though, and Proto Man still doesnt work like he should) but they had to nerf your abilities in order to make the game challenging, and to force you to rely on special weapons. That is a flaw that cannot be overlooked when discussing "best" games. ESPECIALLY when it throws challenges at you which are clearly designed for said abilities. Like shield attackers, whos sole reason for existing is as a gimmick enemy for the slide, or gaps just low enough that Mega Man cannot walk under them, but which would clearly present no problems to a well placed slide.
That kind of thing is something I hate. But It's one of inti's worst habits.
Granted, 9 is more of an offender here with this. But 10 has it's moments too. That's not to say 10 isn't a great game, it is, but I think it is far from "the best in the series", solely due to how it presents it's challenge.