This was my favorite thread on the old r/OSR. So is your favorite system/systems?
I’ll start:
Basic Fantasy - BF gets overlooked a lot because the print books are sold at cost so I think it ends up being a lot of peoples introduction to OSR, but I keep coming back to it. The modules are really well written.
Old School Essentials - Just got the Rules Tome and I love it, though I haven’t had the change to dig into a game with it yet.
Labyrinth Lord. https://goblinoidgames.com/
Just want B/X? Labyrinth Lord
Want AD&D-ish? Labyrinth Lord + the Advanced Edition Companion. (Druids, Monks, Paladins, Illusionists, Gnomes, Half-Orcs, Half-Elves, Rangers, and Assassins, oh my!)
Want Gamma World? Mutant Future.
Want Planet of the Apes? Apes Victorious.
Want Star Trek? Starships & Spacemen
Want GW + AD&D? Labyrinth Lord+ the Advanced Edition Companion + Mutant Future.
Want Full-Knuckle Gonzo? Labyrinth Lord + the Advanced Edition Companion + Mutant Future + Apes Victorious + Starships & Spacemen.
Is it as pretty as OSE? Nope. But you can download Labyrinth Lord + the Advanced Edition Companion + Mutant Future without art for free. If you want art, you do have to pay. They all work with each other.
LL https://www.drivethrurpg.com/product/64331/Labyrinth-Lord-Revised-Edition-noart-version?cPath=2033_6311
AEC https://www.drivethrurpg.com/product/78524/Advanced-Edition-Companion-Labyrinth-Lord-noart-version?cPath=2033_6311
MF https://www.drivethrurpg.com/product/56282/Mutant-Future-Revised-Edition-noart-version
Labyrinth Lord introduced me to the concept of OSR back in 2009 or so and will always be special for that reason.