OD&D (1974).
For one thing, there are almost no rules. It's all rulings. I don't have to look up a bunch of stuff in a book during play, nor waste my time memorizing rules. It's all on-the-fly adjudication. For example, two sessions ago an evil shaman tried to overbear one of the characters and drag them both down into a sacrificial fire pit. I told the player what the shaman was trying to do... leap on him and drag them both over the side. "What do you do?" I asked him... a favorite thing of mine to ask in a role playing game. And at that moment, he could have said anything. He could dodge out of the way, lock into a grapple with him, try desperately to strike the fellow with his weapon, try a spell or any item... anything. And I would adjudicate it as it happened. Very free.
Another big reason is that the game can be tailored to each Ref, and even with a single Ref to each setting. How does spellcasting actually work? Do clerics have to have spell books? Do clerics have to have an *alignment* before 7th level? (The interesting thing here is... traditional clerics worship a god or pantheon and get spells from them. But maybe a junior cleric is a cynical mercenary, and the spells are just ways of manipulating the gods....) It can be however you want it to be. I take joy in the thought of taking the bare skeleton of OD&D and adding the flesh custom tailored to each individual campaign setting or adventure. I can write a one shot that uses the rules in one way, and run a campaign that interprets the rules in a totally opposite way and it's all good... and not confusing because the rules themselves are so sparse, and the action of the game comes not from the rules but from the Ref's rulings.