Let's get this going.
What's your favorite OSR?
Ones I have, * means played.
OD&D * AD&D * Basic Fantasy * Castles & Crusaders Old School Essentials (OSE) Hyperboria OSRIC Advanced Labyrinth Lord Swords & Wizardry * Pathfinder 2e Dungeon Crawl Classic Tunnels & Trolls
My go to is AD&D...
These days I don't really run a system per se. I like a lightweight frame that I can patch other bits on to as I need them.
Whitehack, Dungeon Crawl Classics, Lamentations of the Flame Princess, and especially Pariah are all ones that had a significant impact on how I run games now. There are also several games outside the OSR space that have likewise influenced me heavily.
Though if I am evangelizing it will often be off the back of Cairn because then everyone can have a copy.