A while back I called Swords & Wizardry Complete the best clone. This is still true. The best OSR game is a different matter entirely. That title belongs to Worlds Without Number.
Firstly, my problem with Worlds Without Number is it's a chunky 400 page book. I think it should have been divided into two easier to handle (roughly) 200 page books. And that's about it. After a long enough time I have realized that 256 is the maximum size of an RPG book before it becomes unwieldly.
From a GM's perspective, WWN is the best OSR because it's the only OSR that actually provides the tools to generate a campaign. All other OSRs simply offer words of encouragement or a baked in setting. WWN offers a semi-baked setting with hundreds of pages of building materials to make it's setting your own.
From a Player's perspective, WWN is the best OSR because it's the only OSR with an ultimate level of character customization. The customization itself was partially stolen from True20 (basically the only good thing about True20), the three class mix of Warrior/Expert/Mage plus specific rules on mixing those as a part-class Adventurer. Later in the book there are other classes that can also freely mix with these three core archetypes. Even later on, the game shows you how to build traditional classes that are missing like the ranger, paladin, druid, monk, etc. These classes are built limited and balanced against one another, so druids can shapeshift OR have a pet, not both.
Reading Worlds Without Number is like staring into the eye of god. Only game I've ever owned that made me want to be a better gamer. The GM chapter doesn't just tell you how to make up a world, it tells you how to run a session and how to think. This is both brazen and correct - WWN requires thinking correctly to use it's tools to their fullest extent to generate a fulfilling sandbox. This is the first RPG I've seen that, if two GMs are both running the game correctly (and there aren't any shit players), could actually be a consistent play experience from table to table and session to session. This game is a full rebuke of the idea of rule 0, and appropriately does not include it. You don't change Worlds Without Number, it changes you.
Should be mandatory reading for anyone in the OSR. 10/10 best game in the OSR by a country mile. Stars Without Number is probably similarly brilliant but I'm not interested in Sci-fi. Science Fantasy is the farthest I'll go.
