This is really tough, I guess I don't really have a favorite. I could go with anything from Oriental Adventures and above here.
I really really like the art in the PH, DMG, DD&G, MM, FF, and MMII. MM has the Trampier goodness and the DSC devils and demons, FF has the Nicholson weirdness, and MMII has the fantastic non-silly Holloway stuff. MMII really develops the fiendish hierarchies fleshing out devils and demons but also stuff like the demodands and daemons plus celestial types too. Deities and Demigods art is top notch and mostly covers for the paucity of the narrative descriptions accompanying the cool but mostly unusable statblocks, the 2e Legends & Lore and FR god books are my favorite god books in all of D&D, but the 1e Deities & Demigods has better art. The PH and the DMG are just atmospheric, I love the evocative nature of the party seeing the magic mouth.
The monster collections are all cool, each has really stellar standouts.
OA introduced really cool martial arts for any character, so you did not need to be the weird 1e monk based off the crappy thief class chassis but with a lower d4 HD to have decent martial arts techniques in the game. I really liked that as an aspect of AD&D.
In the end I am going with the MMII, the Holloway Demon Lords and Archdevils really do it for me.