It ranks comparatively low for me compared to other "hardest Mega Man" games like Mega Man 1, Rockman & Bass, Mega Man X6, and even Mega Man 10.
Honestly I don't even find 10 to be hard. Even with easy mode turned off, MM10 always felt really fair to me. If I died it felt like my fault, as opposed to the game just being cheap. I don't die super often either, at least until the Wily stages. MM9 on the other hand feels cheap to me. It's design comes off to me as a "trial and error" progression style often where you have to die, memorize what killed you, and avoid it next time. A lot of the time I just felt compelled to cheat it by using Rush Jet because I was sick of it's crap.
You can brute force it by buying a bunch of items in the shop and using those in combination with Rush's abilities, but I don't count that as the game being fair or easy. I count that as being a really cheap, difficult game that's easy to cheese. I mention because I've seen people defend MM9 as an easy game based on the ease of being able to cheese it, and I really don't think it's a great defense.
Some of the classics sometimes gave me a similar feeling, especially early on. MM1 in particular is littered with all kinds of BS that punishes a new player. MM9 was something of a series revival though, it was meant to be a return to form. In that light, it feels rather regressive. It returns to it's roots for all the wrong reasons while stripping away most of what made some of the more recent games of the Classic series more tolerable. MM10 feels to me like a correction of that, at least mostly. The one complaint I do have for MM10 is that it still doesn't give Mega Man the Slide and Charge Shot. Even with Proto Man being squishy, he feels superior to Mega Man to me in both games, because he has both abilities plus a shield.
If MM11 had Proto Man and Bass as playables, it would probably be my favorite game in the Classic series. It does so much right in regards to fixing the issues with the previous two games. That's the one thing I feel is missing. I wouldn't even mind if they were post-game unlockables with no story or something, the lack of them being here just makes the game feel like something is missing after playing through 10 so many times.
Overall, I know that MM9 had huge ripples not just for Mega Man, but within the games industry overall. It could singlehandedly have the finger pointed at it for the mainstream trend of games being made in an 8-bit art style to cash in on nostalgia (Not that I'm knocking it for that, I quite like the art style myself, even if I prefer 16 and 32 bit graphics over it). Much of what we have now wouldn't exist without it. Especially MM10 and 11, which I love. However, that aside, I still can't help but overall dislike the game. That's why in addition to the knowledge that we got it instead of ZXC, I find it to be a really hard game to forgive.
Anyway wall of text over. I just felt like clarifying my viewpoint a bit, don't want to derail the thread too much.