1. 1e dmg- tons of brilliant, helpful information; can be read for pure enjoyment (and i still enjoy reading it today).
2. 3e manual of the planes- reads like a fascinating encyclopedia.
3. 3e draconomicon- incredibly creative, incredibly detailed book on dragons.
Blood Enemies: Abominations of Cerilia - The book on Awnshegh for Birthright. I think it's one of the best books for that setting, which is one of my favorites to begin with.
The Book of Nine Swords - Woo, weapon-swingy types that don't just say "I power attack" each round!
Oriental Adventures (3e) - And woo, Wuxia! A significant percentage of my 3/3.5 characters thereafter had Tumble, Balance, and Jump scores in the 30s.
Minis would be #1 on my list too. I might never have even noticed the game if it hadn't been for a big gorgeous display of minis in a toy store way back when. Nowadays when I don't play anymore, I still enjoy painting minis.