1. Strike Force for Champions 4rth Ed - a fascinating glimpse into a wonderful personal campaign world by the best Champions writer ever.
2. Traveller original books - Everything an RPG should be: elegant and self-contained in one package.
3. Call of Cthulhu, any edition. Again, self-contained. Great and unique RPG.
4. Cardboard Heroes. I mourn that no more are being made when it's obvious that they'd still sell well. I assume the Fiery Dragon counters sell well since they keep making new ones, so I'd think that means these would sell well again; why not re-issue all the old sets and more more new ones?
5. City State of the Invincible Overlord by Judge's Guild. Not the boxed pretty edition, the original vellum map and newsprint book in a ziplock baggie. Hours and hours of value in that one package, and years ahead of it's time. It remained pretty much the only real city book for a vast span of time, too.
6. Empire of the Petal Throne. Unique, but I never got it until years later because it was (at $30!) tremendously expensive (what with the vinyl maps and everything). I wish someone actually played this where I am.
7. RuneQuest. Ah, for the days when Second Edition RQ ruled the roost as far as providing a fantasy alternative to D&D. The first game (other than EPT) that came with this huge background and unique world setting. Lots of support for it, had it's own magazine, etc...