Castlevania: Symphony of the Night. The game so good it created the term “Metroidvania” even though the formula was basically “Metroid”.

i can tell you the one that surprised me the most: Yoku's Island Express! utterly adorable pinball metroidvania. you're a little dung beetle pushing a big ball around to deliver mail.

i find that there is so much focus on dark and dreary in the metroidvania genre, which makes sense considering the roots of the genre. me, i get enough of that in my daily life. i want colorful and full of curiosity. the ori games are good for that too, as is supraland, but i don't know of many more.

It's a little old and more Metroid- than -vania: A Robot Named Fight

Ori series, specifically Ori and the Will of the Wisps. It's my absolute favorite game of all time. It's got the amazing visuals and insanely good OST, crisp and satisfying platforming and combat, engaging and unique bossfights, incredibly gripping story, good difficulty settings and going for 100% is rewarding and not nearly as annoying as in many other games. While it may stray a tiny bit from the classic Metroidvania formula I think it still does it incredibly well.

I haven't played that many, but I enjoyed Ori and the Blind Forest and Carrion.

I adore Supraland, which is more of a Zelda/Portal/Metroid mix.

A few other good ones that come to mind:

Axiom Verge

Bloodstained: Ritual of the Night

Guacamelee

Momodora: Reverie Under the Moonlight

Rabi-Ribi

A Robot Named Fight (it's basically if super metroid was a roguelike)

Touhou Luna Nights

Cave Story

Also, the Castlevania Advance Collection is three GBA metroidvanias, definitely worth getting if you wanna play/replay any of those.

Castlevania: Aria of Sorrow

Played a lot of these in the last two years, with Ender Magnolia and Nine Sols being my favorites among them.

All-time I think I'd still go with Super Metroid, despite its age and having completed multiple playthroughs I still end up playing for hours anytime I boot it.

Honorable mentions for Rabi-Ribi (don't let the cutesy anime artstyle fool you, this is a fantastic non-linear game with some of the best boss battles in the genre), the recent Momodora games and the Team Ladybug games (with Touhou Luna Nights being my favorite of the three).

Sundered: eldritch edition

I haven't played very many, and these might not be pure Metroidvanias, but I recommend:

The Messenger

Pseudoregalia

Tunic

Supraland

I was shocked to see no mention of Ori in any of the comments here before I realized you had already recommended it in the OP. I have no issues with seconding that, the Ori series is really good.

Rogue Legacy 2, if it counts. Otherwise, Axiom Verge.

I haven't seen Shadow Complex on here yet.

It's an older title (2009) and even the remastered version is from 2016.

Still one of my favorite Metroidvania type games.

https://store.steampowered.com/app/385560/Shadow_Complex_Remastered/

Super Metroid will always be my favourite. I also enjoyed Axiom Verge and Bloodstained ROTN.

I generally like shorter Metroidvania like Iconoclasts and some of the Shantae series or even Yoku's Island Express which has some backtracking.

The Metroidvania tag is a bit like Dubstep in music, a lot of those Dubstep tracks aren't Dubstep but they can still be fun for some people.

La mulana 1 and 2, animal well for puzzles.

Metroid dread, highly recommend for combat.

Dead cells and rouge legacy two for rouge lite bits

terraria for minecraft + metroidvania

time spinner, bloodsworn for more typical metroidvania

the old castlevania games are on steam, so there is that

almost metroidvania suggestion is Outer wilds. Progression based learning. just not in a 2d plane and no fighting.

Just so it's not forgotten to time I want to recommend AM2R (Another Metroid 2 Remake). It's basically what if Metroid 2 was made in the Super Metroid engine.

I'd recommend any of the 2D Metroids, but if you're going to play Metroid 2, that's the version to play.

A bunch of good ones have already been mentioned (SOTN, Nine Sols, Super Metroid), so I'll just mention some that were missed.

Castlevania: Dawn of Sorrow (sequel to SOTN, just as good if you can get past the DS touchscreen BS. The new remaster might get rid of it)

Bloodstained: Ritual of the Night (made by Iga, creator of Castlevania: SOTN, Dawn of Sorrow, and others)

Salt and Sanctuary

The Messenger

I don't know if it's "favorite", but an old classic was Aquaria. It had a surprising amount of content, even to the point of several secret bosses, and an absolutely excellent soundtrack. Sadly, the main guy behind it is dead now. Ori was also fantastic.

Metroid Prime. It's the only series I know of that is fully in the genre and is also not a 2D platformer.

It's insane to me how many Souls Likes could be 3D Metroidvanias if they used special powers to clear obstacles instead of just keys or random triggers to unlock new areas. That's really the only thing I see that separates the genres.

Sekiro is closer than any other, but it's mainly just 1 thing you don't have in the prologue (the grappling hook). I would love a game that is 3D over a 2D platformer, but also has the unlockable traversal tools the way Metroid or Syphony of the Night had.

Ghost Song is one of my favorites that doesn't look like its been mentioned yet.

Batman: Arkham Asylum. It doesn't come up a lot, because only that first game is a metroidvania and Arkham City might be most people's favorite in the series, but it absolutely counts. I love Arkham combat. It's better in the sequels due to some slight tweaks in game feel, but that combat in a metroidvania is just excellent, and the game is just so well paced. It's a shame what WB did to that studio.

Aria of Sorrow

I haven't seen it mentioned and granted I've not played it but I've watched both be played and Blasphemy 1 and 2 seems really, really good.