There's....so many genres out there, and so many games fitting into them, it's impossible for me to say just one.
I'll throw a few out there:
Mega Man 7
The first 16-Bit entry in the classic series is a very strong one, and my personal favorite. The game's formula had already been experimented with a bit in the last few titles (as well as one or two of the GB games), but 7 is where the series really started to get creative.
Sure, Mega Man X was a much bigger leap in the genre than 7 after it, but solidified itself as its own series, allowing both X and Classic to evolve in their own ways.
7 has gorgeous visuals and sounds, it controls nice and tightly while retaining that grounded feeling of the classic series (as opposed to the slightly momentum based platforming of X), and offers more utility to your special weapons than any other classic game to date, as well as X1 before it! The stages open up with new paths or change around you as you use your copy abilities where appropriate, and that's just...so fun to play with when exploring the levels for secrets.
Doom 64
Midway did Doom better than iD did, and I'm not even kidding. With more advanced levels than the classic games and Final Doom episodes prior, the atmosphere and map flow are master class. The game also takes the best of Doom 2's gunplay and, for the most part, beastiary to make for a very balanced experience where you never feel out of your element, nor do you feel overpowered.
Marvel vs Capcom 2
This was what truly got me into fighting games. I remember being entranced by it as a kid at a somewhat local arcade. I had played some fighting games before, but MVC2 is what lit the spark in me to get truly invested in the series.
Sonic Adventure 2
SA2 has the best 3D platforming in the Sonic series, in my opinion- it just feels so good to control. There isn't a bad song in the soundtrack, and the story is that kind of intense, albeit a little hammy, goodness that I love out of the series.
The Binding of Isaac: Rebirth (and its expansions)
It's weird to think this game was something of a coping mechanism for me at a period of time. Even past that point of my life, this game has continued to be one of the most important games I've ever played, one that I always find myself coming back to for a challenge posed by the game or myself.
If you can get past its absolute grodiness and macabre motif then you'll be in for easily the best rougelike out there. Even the base game will warrant you hundreds of hours.
I cannot recommend it enough.
Blood
The best Build engine shooter, and one of the best of the genre. The gameplay is brutal, with little room for error, but it is so rewarding to plow through when you find out when to take cover, be methodical with your dynamite (which is the most satisfying explosive in an FPS) or go in guns blazing.
The motif is a love letter to classic horror and slasher films, with grimdark environments, visceral gore, and a bit of dark humor to laugh it all off. You also have the best protag in an FPS: Caleb, an undead cowboy who's out for revenge against a demonic cult leader. Stephan Weyte's voicework for him is Oscar worthy.
Fantasy Zone
My favorite Sega series that isn't starring a blue hedgehog. The cute and colorful aesthetic always puts me in a good mood, and the gameplay is a nice subversion of the horizontal SHMUP genre. It's one of the few that I can think of that can be played for a best time, rather than just for score. I've spent a lot of time optimizing routes for time, as well as learning funny little parlor tricks to show friends, some of which have been exposed to the game through the Yakuza series.
Fantasy Zone is incredibly difficult, especially when you cannot continue by default, but that makes it all the more exciting to get a winning run through it.
There's a LOT more I could list, but these are some (but not all) that are true Miles-core games.

