Here's a list of my ten most important OD&D resources in my opinion, ranked mostly in order but not totally. (My order changes somewhat depending upon my mood)
1. Core white box
2. Greyhawk supplement -- thief and paladin, expanded spell list, expanded magic list
3. First Fantasy Campaign -- inspiration, examples of early campaign design
4. Best of the Dragon, V1 -- classic articles from Dragon and SR, inspiration, ranger
5. Ready Ref sheets -- random tables for random reasons
6. Blackmoor supplement -- assassin and monk, example of early module design
7. City-State of the Invincible Overlord -- this is my go-to city for every campaign
8. Chainmail -- mostly the Fantasy Supplement, historical and inspriational
9. Eldritch Wizardry supplement -- druid
10. Gods, Demigods & Heroes supplement -- material on Conan, Elric, etc.
Something like that. As I said, my order may vary occasionally. There are also a few other things that I sometimes use like the Monster & Treasure assortment, World of Greyhawk folio (AD&D), monochrome AD&D modules, AD&D Monster Manual (pre-AD&D) hardback Arduin Trilogy, and the like.