My thoughts:

Mega Man 1 is about as hard as I remember it and about as badly designed as I remember it. Not a whole lot to say about it. There's only a few stages I like, and you can really tell that this is the first one. Sloppy slippery controls, overly vertical level design in a game where you don't really get much vertical control (elec man's stage), and a really annoying boss gauntlet (without using glitches). Guts man in general just always felt kinda janky to fight and I don't like most parts of this game tbh, but it's forgivable because it was basically a prototype for the others.

Mega Man 2 is probably not as good as you remember it being gameplay-wise (at least IMO). Boobeam trap is still one of the worst bosses in the series, and if you forgot about it due to PTSD, it's the one where you have to hit walls with crash man's bombs and then hit the individual things shooting you with another bomb, but the thing is that you barely get any time to do it without abusing the invincibility glitch and you only get exactly enough crash bombs to do it. If you mess up once, you just gotta kill yourself until you game over cause there's no way to get energy back. Flash Man's stage is another one of those annoying slidy stages that reminds me too much of Mega Man 1 in general. The controls still have a bit of momentum in 2 as well, which is kind of a general issue I have with it. Some stages are extremely tight buster only, like Quick Man's, which I can only really do cause I memorized it. Bubble Man's stage is particularly annoying, as most of it is just covered in spikes and not much else. It's definitely doable but stands out to me as an overly boring and underdesigned stage. Metal Blade also breaks the game and so many bosses are weak to it this game's balance just feels out of whack. It's still fun, and the soundtrack holds up as everybody knows, but I still feel like it's the second weakest NES title in terms of actual design (Mega Man 1 being the worst).

Mega Man 3 is excellent in almost every aspect IMO and the strongest of the six. While Rush Jet in this one can basically go anywhere and cheese a lot of things, the level design if you do things normally is very well done, and this is the game where they refined the controls to the way they would be for basically the rest of the series. No slipperiness at all, and you can also slide!! I kinda also like not having a charge shot, since hearing the charging noise all game gets pretty old. It's a nice feature but I definitely don't miss it. This game has all the robot masters from Mega Man 2 at the end, but you have to beat them with your Mega Man 3 weapons. The Doc Robot stages that contain these are very difficult (especially if you don't use E-tanks!) and they still feel very fair. It feels appropriate for an endgame challenge. While some feels the game drags on because it ends up being one of the longer games in the series in terms of the different stages you go through, I feel like it's all fun so I didn't mind it. The soundtrack is pretty underrated and really great. The series owes a lot to this game, from the control refinements and additions to the introduction of Proto Man (or Break Man as he's known in this one), and I feel like it doesn't get enough credit because of Mega Man 2.

Mega Man 4 is pretty solid, but you can definitely tell this is when they started running out of ideas for robot masters. Dust Man??? Ring Man??? I get that there's only so many themes you can have, but that's just lame, sorry. Gameplay wise it is pretty good, and not really too easy or too hard. Dr Cossack being the main villain but not really is a trend they continued with 5 and 6, with 5 having a fake Proto Man and 6 having a fake Wily in Mr. X. It's alright, but the plot twist is pretty stereotypical and the stages are not super noteworthy, other than the final Cossack one, in which he operates a crane machine and tries to damage Mega Man by grabbing him. Pretty amusing overall. The control is still good and charging feels natural. Toad Man is easily the worst robot master ever made, though. He doesn't even get a chance to do anything if you hit him and then keep sliding under him, even with regular ol' mega buster shots, since any damage interrupts his only attack. The new weapon menu is pretty good though, and not having to go through different pages is a welcome change that would persist through all future games. You get a balloon that's basically item 1 from Mega Man 2, and a grappling hook that can drag you up to the ceiling to reach things. None of them are necessary to beat the game, and they're hidden to begin with, but they make some of the more annoying level design a lot easier. Mega Man 4 has even more of a habit of spamming spikes everywhere than any past game, so it's nice to be able to maneuver around it. There's one Wily Stage in particular that is really annoying with spikes both on the ground and the ceiling, but some of the other aspects of the game are more easy than usual, so it kind of balances out. Wily himself has a form that's a complete joke because you can stand still and it can't even reach you, and overall he's kind of a pushover in this one.

Mega Man 5 is kind of all over the place, but it's pretty fun. Gravity Man's stage has weird parts where you walk on the ceiling, Most of the other robot masters aren't that noteworthy, other than Crystal Man's stage, which is probably one of the worst stages I can think of in the NES games. There's these things that shoot crystal shit from the ceiling and their pattern is completely random so you can't wait a certain amount of time and hope it doesn't hit you. You just gotta jump and pray it stops while you cross, and if you mess up you die because they're all over bottomless pits. It'd be okay if there were two or three, but there's like 20 of these things. You basically need to do Star Man's stage beforehand to use his shield, which can block one shot before it goes away while you jump over. It's not always good enough to save you but way better than nothing. Oh yeah, and Wave Man's stage has a jetski segment where you can move around, shoot, and jump, among other things. It's pretty fun, and I wish they'd try more vehicular based segments in the classic series, but it also consists of half the level, so they kinda didn't change it up enough IMO. At least there's a novelty of fighting a miniboss while on the jetski. There's letters you can collect to spell out Megaman V. If you do this, which is pretty easy since they are all obvious, you get the robot designed by Dr. Cossack after the events of 4 named Beat that basically flies over to stuff and fucks it up for you without you having to do anything. It does use weapon energy, but it makes things pretty easy overall. On the final form of Wily, this is overly apparent, since you can just stand there and let it win for you basically, only having to jump occasionally. It's pretty decent and the soundtrack is a good step up from the downgrade 4 was after 3's soundtrack.

Mega Man 6 is RIDICULOUSLY easy, and that's probably the most notable quality about it other than coming out after the NES was well past its prime. I'm not sure why I thought of 5 as the easy one, it's probably on par with 4 and vastly more difficult than this one. Despite Capcom's concerns over the game's sales potential, and probably because they held a contest where some North American fans designed 2 of the robot masters in the game, Nintendo insisted on publishing it anyway, and it's alright but some of the stages are just nonsensical. Centaur Man's in particular makes no sense at all. Would you associate a stage that takes place half underwater with robotic jellyfish and pelicans with a centaur?? I sure wouldn't. You get rush adapters in this one, one of which is a jetpack that replaces rush jet which is pretty useful but very limited in flight time, and the other of which is a power upgrade that makes all your basic shots do 2 damage, allows you to destroy some otherwise impossible to destroy things in exchange for a limited range and no sliding. You pretty much just punch shit and fuck shit up in this mode, it's pretty good. Mr. X is the main "villain" in this one, but it's just Wily yet again. Tomahawk Man has a very stereotypical Native American design complete with feather headdress and Flame Man has a design with a turban and curved boots in a stage filled with oil. I'm not sure these would fly today, and they're kinda weird. Anyway, all the X stages and bosses are pretty easy to go through in no time, and honestly Wily/Mr. X don't put up much of a fight at all. Beat is back in this one with only four letters to collect, some of which are behind alternate robot master doors. When you finish some of the stages, there's a "fake" and "real" robot master door you can go through, with the real one often requiring rush jet or the power adapter. Both give you credit for beating the stage, but only beating the real robot master gives one of the four letters. Beat is basically the same as the last game, but honestly I never even used it. The game just throws E-tanks at you and I had 9 almost all game, even when I used like one per stage, it's kind of ridiculous. If someone were new to the series I might even start here rather than Mega Man 2 "Normal" mode in US (which didn't exist in Japan and is basically easy mode). Maybe Mega Man 10 easy mode would be better, but I'm not really sure. Mega Man 11 is actually a good deal harder than some of the past games IMO. Anyway, overall the game is pretty easy but pretty good. Even though I just beat it again, there's not a whole lot of memorable things about it though. The soundtrack steps it up a tiny bit from 5, but that's about it. The game really didn't have much of an impact on the series as a whole either, other than the rush jetpack recurring in a couple of games. It's alright but really lacking the feeling of overcoming a difficult challenge the series is known for.

And that's about it! I think I'd rank the games in order from best to worst 3 > 4 > 2 > 5 > 6 > 1 if I had to do that kind of thing, though I consider 1 far worse than any of the others. 2 is still good to me, but it's still unrefined and makes a good amount of questionable decisions, so I honestly might even prefer 5 over it. I think 6 is more solid controls/feel in general wise as well, but it's just way too easy for me to put that high.