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.
There is a monster book, but it’s a book about generating monsters rather than using existing ones. This is a deliberate choice that’s explicitly stated in the guidance Raggi gives to writers interested in submitting new content you can find posted… uh… somewhere.