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.
For players that want crunch: AD&D 1e (if they can’t get the books, then OSRIC is the AD&D retroclone).
For when I want to make a setting and need D&D rules: OD&D - the original 3 books. I need little else. If folks need a PDF, the Swords & Wizardry White Box 3rd printing is free and handy.
For when I want to make a setting and need sci-fi rules: Classic Traveller - I’d use the original 3 little black books from 1977, but anything from the classic era (1977-1985, pre Mongoose) works. If folks need a PDF, the CT “facsimile” is the 1981 version with errata applied.