1e DMG?
Best. Book. EVAR.
The only thing that I have a dislike for is the unarmed combat rules (but we mastered them anyway, until I came across "Finish Fights Faster" in
The Dragon, which became the Gold Standard for unarmed combat in our games); everything else (organization, prose style, tables, art, etc.) is golden, from the cover to the afterword and beyond. .
Diseases?
Cool.
Potion Miscibility?
Orgasmic (although, I once tried to trick a PC into blowing himself up

and he wound up with a permanent Cloud Giant Strength effect.

Never again did I intentionally try to get a PC to blow himself up

).
Random dungeons, properties of intelligent magical swords, funky artifacts?
Awesome.
Dungeon-dressing tables, herb lists, reputed magical properties of gems?
Cosmic.
I had it virtually memorized from cover to cover; what I couldn't recall on the spot, my fiancee (now wife) could.
Who needs a functional index?