Holmes Basic Set that I've owned since it was released (and I was in 5th grade). Plus a bunch of Dragon magazines, 1e PHB, DMG, MM, Fiend Folio, and most of the original adventure modules, a bunch of Judge's Guild supplements, adventures, maps, etc., most of the Grenadier minis (plus a good number of Ral Partha), World of Grayhawk Box Set...But most of all, the notebooks full of notes, scribbles, homemade adventures, character sheets, etc. All from when I first started playing.
I've since picked up all the white box pamphlets (minus the box) and a 3rd edition of Chainmail. Funny thing is, I remember seeing all of those at the game store where I bought my stuff, and thought they were cool from an historical perspective, but chose to spend my hard earned allowance on the newer stuff.