Just gonna drop every great game I have on Steam that qualifies and hasn't been mentioned in this thread/isn't obvious. Games I put pictures of are among my highest recommendations:
Platformers
>Kero Blaster
Pixel's best game in my eyes, and one of if not the best in its genre (which is saying a lot). Genuinely insane that it's not even 5MB.
>Gravity Circuit
Indie GOTY 2023, think Mega Man X/Zero with more melee combat. Striking artstyle and fantastic music, plus a really great boss lineup.
>Decline's Drops
Hand-drawn game that controls similarly to Smash but has level design closer to DKC & mainline Kirby. Extremely charming with a distinct style despite having such clear inspiration.
>Pangolin Cassowary
3D platformer where one character kicks another like a ball in order to get them to different places. Kind of reminds me of Kirby's Dream Course.
>Bat Boy
The only platformer I've seen that I'd say was primarily inspired by Shovel Knight, though it's a much faster-paced game than Shovel Of Hope with a lot of abilities designed for speedrunning levels rather than being necessary for progression.
>Bionic Commando ReArmed
One of the great but sometimes forgotten remakes before remakes were oversaturated. Kind of shocked how small this one is actually, not even 400MB.
>Spark The Electric Jester
A game that is equal parts Classic Sonic, GBA Sonic, Kirby, and Mega Man Zero. Really fast Sonic-style platforming with some solid combat mechanics and copy abilities you can swap between two of. Has two 3D sequels that are more Sonic Adventure-esque but minimize the copy abilities over time (and are well over 1GB).
>Bloodstained: Curse of the Moon 1&2
Classicvania games Inti Creates put together to promote Bloodstained Ritual of the Night. They're kind of like Castlevania III but with instant character swapping instead of slow menu swapping.
>Mechstermination Force
Contra + Shadow of the Colossus with a sense of humor. Some really fun concepts for bosses. Same dev who made the Gunman Clive duology (also very small action platformers that are good fun) and was actually on the BC ReArmed dev team before going indie.
>The Fancy Pants Adventures
Technically Early Access, this is the original Fancy Pants creator bringing the original games to a modern standard and adding a bunch of bonus features, updating it as he goes instead of waiting until the whole package is complete.
Action/Beat-Em-Up
>Final Vendetta
Very classic beat-em-up in that Final Fight/Streets of Rage style that has no Continues at all but is well-designed enough to be doable anyway. Outside of sprites that look a bit Neo Geo-esque this could've passed for a very late SNES game in terms of its presentation and design.
>Aztez
2D Action game where you play a simplified 4X campaign on a map trying to strengthen and unite the Aztecs, doing combat to ensure your actions play out, ultimately trying to fight off the Spanish. Really fast combat where you beat all the blood out of enemies so you can sacrifice it to gods to do supers.
>Aces Wild: Manic Brawling Action
A stage-based 2D Action game with really intentionally floaty and fluid movement and attacks that also let you bounce enemies around the stage.
>Way of the Passive Fist
Seems like a beat-em-up but you need to dodge/parry enemies to tire them out before doing single powerful strikes. Really more of a rhythm/reaction test framed like a beat-em-up but positioning does matter at times so you still need to be on your toes.
>Fairy Bloom Freesia
Simple arena-based 2D action with a cute fairy. Same dev team as Sakuna Of Rice & Ruin.
>Super Crush KO
Very pastel "cutesy-targeted-at-adults" style, but it's actually got a really good fluid action game under the hood with pretty demanding S+ Ranks even if just beating the game is really easy.
Arcade
>GIGANTIC ARMY
A slower-paced run & gun where you're a mech walker with 360-degree control over your weapon's aim.
>Metal Slug XX
Something of a forgotten entry in the series, but a pretty solid one. It takes place across a single day on one island instead of being a globetrotting adventure which makes it stand out a bit more, plus there's 7 playable characters.
>HAYAI
Single-screen score-chaser where you just move your character around dodging enemies & projectiles, then drag your mouse across the screen to attack. There's just enough variety across the five characters and style in the handful of assets to keep it interesting.
>Devil Blade Reboot
A remake of a pretty obscure PS1 shmup from a one-man dev who actually works at Vanillaware and made this on the side. The perfect gateway game for people getting into the genre, based on my own experience, because it's gorgeous and has a very attainable skill floor to do well.
>ZeroRanger
Pretty well-known but it's a really excellent and mostly original shmup that actually does some interesting things with its story in terms of how it tells it with the gameplay and mechanics. Incredible OST too.
>Lilac 0
Very recent release, very ZeroRanger-esque game with a big difference being the dash attack you can do with your sword.
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>One Finger Death Punch 1&2
Enemies come at you from either the left or right and you press a button to attack or counter them. Surprisingly these games get a fuckton of mileage out of that. Kind of looks like old XiaoXiao animations when you're playing later levels well.
>Angel at Dusk
Another shmup but one with genuinely insane visuals and intentional sensory overload at times. Lots of flesh and bones from the ships to the backgrounds to the UI.
>Shatter
Probably the best brick-breaker ever made with a killer OST. Make sure you get the original and not the remaster, the latter was made for mobile and then ported to PC.
>Sayonara Wild Hearts
What I can only describe as Pepsiman-like gameplay set to an album's worth of synthy pop music. Barely over an hour long before replays/rank chasing, but definitely look for a deal despite the quality since the base price is a little too high IMO.
>REDLINE CROOKS
Top-down driving shooter roguelite thing, nothing quite like it gameplay-wise with how frantic it gets. Simple charming art too.
>Toxic Tomb
Not on Steam but it is freeware. Master System-styled single-screen action platformer-style game that's just super well-designed. Made by the guy who did Maldita Castilla and a bunch of other great retro-style games (which are mostly freeware). Link: https://locomalito.com/games/toxic-tomb
>Candy Rangers
This one technically isn't out yet but I just adored the demo and the Steam page says it'll come in right at about 1GB. An on-rails shooter where you control four marching magical girls who need to shoot down various enemies at the right time with the right color; it does a lot with the premise just in the demo and it's overflowing with charm.
Puzzle
>Tetrachroma
Tetris, but you need a line to match colors for it to clear and blocks change the color of everything they touch in a straight line to match themselves when placed. Drastically changes how you build yourself up while playing and has a TON of modes, features, and unlockable cosmetic options. Also "Monochrome" mode is just regular Tetris and it's a good version.
>Save Room
RE4 Attache Case as a puzzler; fit all the items in the case, even if it means you need to combine things or put ammo in guns first. Really satisfying even removed from the blatant Resident Evil references.
>KNIGHTS
Super minimalistic game where you just have to move Knight pieces from chess into specific spaces on grids of varying size. If that sounds appealing there's a sizable number of levels that get pretty complex as they go.
>Lumines Remastered
It's a Mizuguchi game, so it's got great music and gameplay synced to it. It's not as good as Tetris Effect but it's different enough to be appreciated in its own way.
Fighting/Competitive
>Your Only Move Is HUSTLE
Turn-based Fighting game that can be used as either a super in-depth game of reading your opponent's options multiple steps in advance or a sick fight scene choreography machine. There's a ton of character mods too.
>Inversus Deluxe
Really smart concept in which you fire bullets to change the color of squares in a grid to increase the area you can move and block off your opponent so you can actually hit them with a bullet to take a point.
>Dead Man's Hand
Russian Roulette with cards where you play as old west robots.
>Dojo Masters
A simplified fighting game with simple graphics but a good amount of depth; every character represents a fighting style and you usually win by getting points for downing the opponent rather than just depleting their life. Very neutral-heavy and fast-paced.
>ROBO OH
A fighting game built around the NES/Famicom in aesthetics and controls with a roster of only mechs.
Other/VN
>Merchant of the Six Kingdoms
Haggling Simulator where you're a traveling salesman who has to bargain with everybody you meet to get the best deals on everything so you can make a profit. Also has a collectible card game you can play with some NPCs.
>Dead Horizon
Freeware game you can beat in 10 minutes that combines a Point & Click with a Shooting Gallery to tell a short story. Pretty much nails what it's going for. The guy who made it is working on a much larger-scale version of the same concept with the same name.
>Endless Monday: Dreams & Deadlines
Cute VN about overworked office ladies and chronic procrastination. Not usually my kind of game but the art and premise are right up my alley; I admit I gave it a look because the guy who made Helltaker drew fanart of it.
>The Murder of Sonic The Hedgehog
For a quick little joke VN made by Sega of America employees on the side it's really charming. Baby's First Ace Attorney with Sonic characters and a genuinely fun little isometric platformer you play levels of intermittently.
>Nina Aquila: Legal Eagle, Season 1
RPGMaker Ace Attorney-like. Very budget but with some charm and some decent ideas, as well as some minigames related to the cases like a card game. Also Nina's design makes my pp hard.
Honorable Mentions (aka just over 1GB)
>Symphony of War: The Nephilim Saga
Indie GOTY 2022. Think Ogre Battle/Unicorn Overlord squad system attached to 00s-era Fire Emblem level design. Gorgeous in-combat sprites, though the rest of the assets look more stock/standard.
>Streets of Fury EX
Really goofy combo-heavy beat-em-up that uses digitized photos for characters and backgrounds. Same studio was half of the team behind Streets of Rage 4 and the upcoming Absolum. This is that game where you can play as Nostalgia Critic and juggle people with his gun.