The rarest RPG I own? Rarest, huh?
No idea. Not as up on the collector's market as I used to be.
I mean, I have the original LLB books from Traveller, the AD&D 1E Deities & Demigods with Elric and Cthulhu, and first edition RPGs that only came out in Japan for crying out loud!
(Looks around with shifty eyes)
I also have the playtest rules for Last Unicorn Games' ICON System Star Trek RPG. I was one of the playtesters on it and a writer on one of the supplemental sourcebooks. The version I have differs in a few big ways from what was finally released.
I read it so many times I used to use that version in play, and players would say, "Adam, I don't think that's right. It says here X, Y, Z.", or what-have-you.
I still do it from time to time. I actually prefer the playtest version truth be told.