If it's a PbtA please expand on your system of choice!
I'll start:
I really enjoyed Sexy Battle Wizards, but I think The Witch is Dead speaks to me more. I like player characters just being little weak dudes and struggling against normal stuff. (both of these are free by grant howitt and 1 page of rules)
I also enjoyed rude detectives, it has a surprisingly juicy dice system for a game that's just 4-5 pages. It's themed around child detective stories.
PbtA I like magpies stuff, like avatar and root, but my favorite until now was Fellowship 2e I think. Just so extremely versatile, and it's a joy to give a lot of narrative control to players.
Last one is kinda cheating: my current favorite game is rules light within it's genre. 13th Age is the easiest 'dnd' I've ever seen, and the amount of dumb rules and bookkeeping it prunes has reinvigorated 20f games for me! But compared to other games, it's still a bunch of rules of course.
Ironsworn is one of the lightest systems I enjoy, it's on the crunchy end of PbtA. Lots of moves so you don't have to rely on Defy Danger that much when doing some perilous adventuring. Novel way of handling "clocks" with their own set of moves. And fully support Co-op and solo play which also ads some moves.
Blades in the Dark is lighter but I don't quite make the setting work for me, its awesomeness have yet to click for me. Its mechanics though - chef's kiss.
@tissek@GataZapata I don't think you can call Blades (or Wicked Ones, mentioned below) rules light. It's core mechanic is super simple, yes, and has inspired lots of lightweight hacks. But Blades has soooo much tacked onto it that it almost feels like a board game to me in the sense that there are just so many moving pieces, and stages of play, and faction play, etc: it feels like a collection of mini-games sometimes.
I know... I just need to find the time and space to delve properly into it. Wicked Ones is already high on my reading list, Band of Blades sure does interest me. Time though...
Lasers and Feelings is so rule-light the rules document is actuallya single page. I played it once and had a blast with my character, who I named Beef McLargeHuge and who got into some weird situations, and mostly solved problems by seducing whoever whatever was convenient.
There are no DCs. Each player chooses a number between 2 and 5 to represent their characters' aptitudes. High means they're better at Lasers "(technology; science; cold rationality; calm, precise action)", and low means they're better at Feelings "(intuition; diplomacy; seduction; wild, passionate action)". The GM only needs to decide which of those two any action is (they need to roll under their number to succeed at Lasers, and over their number to succeed at Feelings), and also whether the character is prepared (based on the situation), or an expert at the subject (based on their characters' chosen expertise), and for each one they get an extra die. The target number is always the player characters' own chosen number.
Tricube Tales. It's the easiest game I've run, and strikes a perfect balance between rules lite narrative shenanigans and a traditional roleplaying game. Uses an easy to manage 1-3 pool of d6s, and all the GM has to do is pick a Trait (Agile/Brawny/Crafty) and assign a difficulty to the challenge (most of the time it's 5). Character creation is pretty freeform, with PCs getting Fate aspect-style Perks and Quirks. You can run just about anything in Tricube Tales. It's a real joy.
I'm also big into FIST ULTRA Edition, a game about paranormal mercenaries set during the cold war. Inspired by Metal Gear Solid and Doom Patrol, FIST is based on World of Dungeons, so it's a 2d6 system, sort of like PbtA with only a "defy danger" move. The heart and soul of the game are the 200+ Traits, which are a sort of combination half-class, feat, starting gear, and attribute score adjustment. When you make your mercenary, you get to pick two Traits, and the ensuing combination is always fun.
I've only played 10-15 or so systems with a a few forays into extremely niche systems, but I've really enjoyed Fate Condensed, far more than I thought I would. My regular group are the ideal audience for it too as we all have a very 'writers room' approach to storytelling.
Unfortunately it's paired with 100+ 5e sessions and no others have lasted more than 5, which is probably where my cynicism about it comes from. Plenty of mine were also one-shots.
Fate is probably about as freeform / fits all as you can get, and compared to other similar systems like GURPS, making the kind of alterations to make it fit Fantasy Vs apocalyptic Vs cyberpunk etc is really easy, but the system remains evocative as you chop and change it.
I'd absolutely recommend it as a one-shot system, although it's also good for campaigns too. Players are encouraged to leave some parts of their character sheet blank and fill it out in the session as they discover who they are, which I think really helps players have satisfying characters for one-shots as they don't get stuck not knowing their characters.
I really like Fate but I've struggled to find a good group for it. I tried to run it for my DND group and it didn't really go well. I think partly because they didn't know the system well and approached it more like DND - very zoomed in on their character rather than the more "writer's room" style that fate can do well.
Does Forbidden Lands count as "rules light"? I feel it does (apart from Index Card RPG it's probably the most rules-light game on my bookshelf). Its cousin game, Dragonbane releases soon (and it is a lovely little game, artistically) and it's pretty damn light.
I love a good rules lite system. Knave and it's hacks, Mork Borg and it's hacks, The Black Hack and it's derivatives are faves but The Black Sword Hack is my #1. Deadly, classless, magic and tech are strange and dangerous. While keeping the rules minimal and easy to grok.
Broken Compass came into my life recently and it's a shame how CMOM fucked up the release, but the game itself is great. Dice mechanics are basically looking for n-of-a-kind where n is between 2 and 5, which is neat because you can use literally any six sided dice that have unique sides. My friend is running some old CoC modules using it and it's been fun so far.