There is one game I know backwards and forwards, own every rule supplement for, and yet never officially played: the Original Edition of Dungeons & Dragons.
|My complete set of OD&D digest-sized rule books|
My first exposure to this edition was a photocopy of Gods, Demi-Gods & Heroes that somebody gave me in the late 80s. It wasn't the entire book; just the Hyborian and Melnibonean mythos. In the mid 90s, I was blessed with luck and found a near-perfect "white box" OD&D set at a comic book store for $10. I picked up Chainmail from a dealer at the San Diego Comic-Con for $9.95. I assembled the rest of the set piece by piece, finally completing with Swords & Spells (invaluable for its definitions of OD&D spell ranges and durations).
I also have a copy of Tractics, perfect for combining with OD&D into a game of Sturmgeschutz & Sorcery!