My near-complete Judges Guild collection. I quit gaming back in my first semester of sophmore year at high school (1984) and sold all my Judges Guild stuff (nearly a complete collection then, too). Fortunately nobody wanted my D&D and AD&D books, so I was not totally screwed when I got back in... three months later. Unfortunately, by then everyone realized that JG was out of business, and nobody would sell me back my JG books! It took me almost two decades to put the collection back together... the gem of the collection, however, is a book given to me by a friend: Bob Bledsaw. Bob gave me one of his original personal house copies of the 56-page Guide to the City State, complete with hand-written notations on the map. It's banged-up, torn, discolored, missing the cover, missing maps (of which I have plenty in all sizes anyway) and in poor condition, but it is a true treasure nonetheless!
My other major gem collection would be my Mystara books. The only Mystara products I am missing, AFAIK, are the Jade Hare and original orange Princess modules, which are too pricy for my wallet these days when it even can be found. My Grand Duchy is on its second layer of tape to keep it together, but it is most cherished.
There are others, worth more money wise perhaps, but those two collections, and the style of gaming and the memories they generate of games with friends over the years, are at the core of my love of the game...