This might be a question best asked in OSR General >>52414491
but it's a bit long in the tooth right now, so it might be a good idea to wait until a new thread gets started, likely tomorrow.
But to address your question, it depends on how strict we're being with the definition of "read" (or "played" for that matter). I've looked over a shit ton of OSR games, and owing to their commonalities with D&D editions I'm familiar with, it's pretty easy to pick up how things work, but I've read almost none of them cover-to-cover.
I'm still a big fan of Moldvay Basic, but if you're excluding original D&D editions, then I guess I'd have to say that I like Swords & Wizardry White Box for the simplicity (and Swords & Wizardry in general for the single-stat saving throws), Labyrinth Lord AEC for simplifying AD&D (even if it's only partway successful), Lamentations of the Flame Princess for innovating in the areas of thief skills and encumbrance (among others), and Castles & Crusades for adopting a unified mechanic and not being afraid to streamline and tweak AD&D (even if the SIEGE engine is a bit wonky, and saving throws are blatzed).