(Also played via Legacy Collection)
Pretty novel and groundbreaking concept for its time but this game is just fine. It's a nice foundation for the rest of the series, but a gawky and unpolished game stacked with cheap hazards, beginner's traps and just plain shoddy level design. The good thing though is that despite all the nuisances and some elements that didn't age very well, Mega Man still manages to be a charming and fun little game with great controls and game feel, impressively detailed spritework and scenery for an earlier NES game (albeit aesthetically dull compared to later games), a nice arsenal of weapons that is just great fun to use and bosses with weaknesses that are fairly easy to make sense of. To put it simply, it's the first game in the series and it absolutely shows.

Also the music in this one is just okay. It's good but it's definitely one of the weaker Mega Man soundtracks to me, at least from the classic series.

(Also played via Legacy Collection)
Much like Sonic 2, Mega Man 2 is an improvement from the original in every way possible.
It adds E-Tanks, the amount of health and ammo that the enemies drop is pretty generous, the level design is pretty great, the soundtrack is an absolute classic, the Robot Masters' weaknesses still make sense, and some of the weapons are godlike. The Metal Blade is the best Robot Master weapon in any Mega Man game, it's incredibly useful given its ability to throw the blades in eight different directions, and it honestly makes the standard Mega Buster nearly obsolete given how generous the game is with its health and ammo like I said before. That makes it sound like the game is too easy, but no... I mean it is the easiest classic Mega Man game, but it did kick my ass sometimes. It's still challenging but it's a level of difficulty that is very enjoyable, unlike the first game which had a lot of unfair bullshit.
But when you reach the Wily Stages the game drops in quality I must say. The stages themselves are okay, but the bosses are mostly pretty awful, especially the Boobeam Trap, one of the worst Mega Man bosses, it sucks ass.
Overall it's pretty awesome, one of my favorite NES games and one I can replay anytime.

(Also played via Legacy Collection)
Many people consider this a top-tier classic Mega Man game but not me. This game honestly pisses me off, because the first half of the game is super enjoyable. The Robot Master stages have great level design, themes, etc. But then you reach the Dark Robot Master stages, four stages that are an absolute condemnation. They crank up the difficulty by a lot and also drop the quality of level design by a lot, not only that but eight recreations of Mega Man 2 bosses that are just garbage. You need to figure their weaknesses with Mega Man 3 weapons but these assholes move too fast and do too much damage to the point that is nerve-racking having to try and guess what is weak against them considering they can kill you in a matter of seconds. Also the weaknesses to the Robot Masters make no sense and the weapons are lame. The Wily Stages are fine but they don't help, this game doesn't recover after those four agonizing levels and the final boss is real bad.
This game is really saved by its first half and awesome soundtrack, as well as the addition of Rush and the slide, because honestly this might be my least favorite classic Mega Man because of everything sucky about the second half (I can't say Mega Man & Bass is the worst because I haven't played it, but I heard it's pretty bad).

Edit: Okay I played Mega Man & Bass now and yeah that game blows

An absolute delight of a game. Mario 64 just shows how exceptionally talented Miyamoto, and Nintendo in general, can be considering that on their first try they nailed the 3D platforming formula that would bring the Nintendo 64 its great reputation despite of its massive flaws as a console.
Mario 64 has what is called nowadays "the sandbox style" of 3D Mario games. It embraces the realm of 3D with open levels filled with areas to explore and little secrets to discover. Every time you enter a level the game sets a goal, the cool part is that there are multiple ways to reach that goal due to the explorative design of the levels and the brand new moveset made for Mario, which makes traversing these creative levels extremely fun. You can get the stars in a level in any order you want, you don't need to get one star to get another star (ok there are some exceptions but still), which is something Sunshine did and it was a big letdown. Here you don't need to finish the game with all the stars, it's all about going at your own pace, explore the levels as much as you want, grab as many stars as you want, it's all about the freedom the game provides you.
I also think this game has aged way better than many people say it did. Sure the camera is a little... terrible, and the Wing Cap's flying controls are very sucky but aside from that I don't really find Mario 64 all that dated honestly. The controls are wonky yet very nice and easy to get used to, and the visuals are mostly nice for a N64 game. I also like that the game doesn't look like a generic Mario game which is a problem I have with future games like 3D Land and 3D World. It does use the same level themes as every Mario game like grass level, snow level, fire level, but these levels always feel like they belong in this game and don't resemble any level from the other Mario games and that is pretty rad.
I have so much of this game stuck in my brain because of how many times I played it, I know most of the levels inside out, I remember exactly how the game sounds, I remember exactly how the game feels, it's such a marvelous fucking video game. It might be my most played video game even if it's not my absolute favorite (but it's not far from that). Super Mario 64 is indeed a big boi.

Remember when Mario went to jail and Bowser spoke actual sentences?

So Super Mario Sunshine is a really fun time but a very flawed game, which really upsets me because this game had everything to be a masterpiece. It's one of the best controlling Mario games; the F.L.U.D.D. nozzles make for some pretty interesting and varied movement either vertical or horizontal; and it sprinkles so much personality with its consistent tropical setting, all the levels feel part of the same world and weirdly enough this thematic consistency makes Sunshine one of the the oddest Mario games out there, in a good way.
But man you probably already know but this game is super rough around the edges in some areas. The numerous bugs and glitches; the godforsaken blue coins; the unability to go at your own pace because you need to get all the shines in the main stages to finish the game which is a massive step down from Mario 64, which gave you the freedom to explore and get any star in any order with multiple ways to do so; a lot of repetitive missions and specific infamous missions that are literally broken with physics and mechanics that completely shit themselves from the lack of polish. Some of these missions are optional, sure, but some aren't, and even if some are optional it doesn't justify the untested and broken levels like the pachinko machine, the lily pad level as well as Corona Mountain, the watermelon and Sand Bird missions in Gelato Beach, and the chucksters in Pianta Village.
At the end of the day Sunshine is still a quality game with loads of fun missions and levels, the best hub world in any Mario game, Yoshi, F.L.U.D.D., the Spin Jump, and a really nice message about cleaning to help the environment. But those problems make the entire experience feel like a major stepdown from Mario 64, but I wouldn't mind replaying this game again. It's good fun but one of the weaker 3D Mario games definitely.

Don't 100% this game, you're gonna hate yourself for that.

I don't really have much to say about Super Mario Galaxy 2 other than it's phenomenal like the first one. It has improvements over the first one and also has personal letdowns compared to the first one.
In terms of improvements, well, the power-ups here are much better, the Cloud Flower is one of the best things ever created; Yoshi is here, which is always a positive; the Starship Mario hub world, while not as big as the Comet Observatory from the first game, does have a lot more to do on it so that's fun. And... I dunno, it keeps a lot of the qualities from Galaxy 1. The phenomenal level design, the gravity mechanics, the fantastic orchestral soundtrack, the smooth gameplay, it's really awesome.
But my biggest issue with this game is that compared to the first it totally feels less like a space adventure and more so like a whimsical Mario game, which is great don't get me wrong, but one of the reasons why I love Super Mario Galaxy is the pure immersion on the vast space atmosphere. The levels in the original felt like actual ecosystems spread all over the universe, where in Galaxy 2 they feel more like videogame levels. Galaxy 2 still has some levels that capture the essence of the first absolutely but it's not frequently. The blue sky theme is more prevalent here and yeah that gives Galaxy 2 some personality and identity but I will always prefer Galaxy 1's emphasis on pulling you into this huge, exciting and beautiful space adventure.
Also the level selection is pretty much a straight line where you have to unlock the next level to unlock the next level and- you get the idea. They could have done something like Super Mario Bros. 3 or Super Mario World and added secret routes to unlock secret stages and make the map navigation feel much less linear.
Yeah that pretty sums up my feelings on this game. It's a masterpiece of 3D platforming that I love and it definitely has improvements over the original but the more happy-go-lucky Mario atmosphere, more forgettable levels compared to the first one, linear map navigation and 99% less Rosalina make this game really not stand a chance compared to the first one. But that's my personal opinion, I will totally understand if someone prefers this over Galaxy 1, it's really good on its own merits.