I've either run or played these all as one shots, and are ones I recommended to a friend recently. I also have links at the bottom to a bunch of free quickstarts for more games to look at that might be good for one shots too.
Blue Rose - AGE system from Green Ronin. Romantic fantasy where royalty is chosen by a magical stag and the rich are appropriately taxed. Cool Stunts mechanic to do extra abilities during a turn, easy system to get used to but has that crunchy feel too. Sentient animal race that forms bonds with people is an instant love every time. Free quickstart.
Heart: the City Beneath - Resistance system by Rowan Rook & Decard. A dungeon crawler where the dungeon is an NPC. Take stress until you can't, go out in a blaze of glory with one time Zenith abilities, such as summon a train to wipe out everything in its path - including you. Heart's the love of my life and the first RPG to make me cry(in a good way). It's earnest and grotesque and something about going in knowing you will End, and choosing it, does something to me. Also you can be full of bees(on purpose). Quickstart is PWYW($3ish suggested) but you should give them 1 million dollars actually.
Bones Deep - Troika! system based by @technicalgrimoire. The other love of my life. Be a skeleton on the ocean floor! What happened to your skin? Not important! Do jobs for witches, get credits from the crabs, join the cephalopods in their nefarious plans. The classes are fun(shapeshift your bones, or carve spells into them), and the mix of horror and humor is immaculately weird. Many random tables and a couple scenarios to make it easy to start swimming walking. Also not free($15 for digital), but worth every cent and then some(and the website is awesome resource!).
Index Card RPG - d20 system from Runehammer Games. It's 'simple enough to fit on index card'. There's actually lots of stuff, but it's easy to get a game rolling fast! Be a gerblin, get a bunch of loot. If you're a chronic low roller like me, a nice mechanic is when you fail you get to put points in a pool to use for later rolls. Probably the easiest one on this list to jump to from DnD. Free quickstart.
Wildsea - Wild Words system from Mythworks(took me awhile to find the name for it lol which isn't important just play it xD). You can be a mushroom person piloting a giant whale bone ship with a chainsaw on the front above a sea of trees. The world is so cool and unique. Health is measured in stuff to lose/break and there's a sliding success mechanic(you succeed, you succeed BUT- etc.). Free quickstart.
Tales from the Loop - Year Zero engine by Free League. Be kids in an alternate 80s-90s Scandinavia(or US) setting trying to navigate growing up alongside strange tech and apathetic adults. I really like how they tweaked the system to make it fun being kids for this, with iconic items and luck points, and it's very collab focused. I used this scenario as a one shot. There's a starter's set for $4.99.
Old Gods of Appalachia - Cypher system by Monte Cook. Face horrors man-made and monstrous in the haints and hollers of an alternate 1930ish Appalachia. Based on the podcast(don't know anything about the podcast? Even better imo). Cyphers are neat one-time use items you find during game to make you stronger/invisible/etc. Free quickstart.
Under Hill, By Water - OSR hack by Rise Up Comus. Hobbits! Okay, "halflings" for copyright but...it's hobbits. While some are off saving the world, your greatest worry is finding the lost pig before your wife realizes it's missing. It's not a boring life, just a quiet one. Random table generators for Events to happen each season make this easy to pick up, or come up with your own! $10. We loved this one so much we turned it into a mini campaign.
Mörk Borg - OSR-ish by Free League. Last but not least in the amount of shelf space in my room! The basic rules fit on one piece of paper, but the books are packed with amazing art and delicious disgustingness. It's easy, quick and deadly, with so many random tables for generating baddies, loot, etc., and usually each version of it has a doomsday countdown calendar with horrid new things to add, and then a final horrible event to end the game. It IS very dark and gloom, and I don't normally play it as rough as the OG, which works out fine! You can get a free version of the core book on the website linked, and tons of other free goodies there. My current fav hacks are Pirate Borg and Ork Borg.
List of free Quickstarts I found(and some I'll be playing soon!):
jacobvanloon
ellievsbear
stormborns
dededos
2087
blakekathryn
tangentwithtanner