I don't really know what good stuff 1st edition has that I may be currently missing out on.
Basically splat. Seemingly, the lion's share of in-house creativity and scope was saved for AD&D, as were contributions to the magazines. D&D and Mystara seems to have been mostly Frank Mentzer's rules, plus freelancers making the supplements.
With regard to raiding AD&D, I think the motherlode is the 2E Wizard's and Priest's Spell Compendiums and Encyclopedia Magica books, which compile about 20 years of material across
all D&D and AD&D editions. Also, basically all 1E monsters eventually saw their way into the 21(?) Monstrous Compendiums, but lavished with more fluff detail. Collectively, these books represent thousands of weird and wonderful spells, magic items and monsters, the like of which will probably never be seen again.
These resources weren't fully appreciated at the time of release because 2E was at that time creaking under the weight of umpteen broken kits and players option books, plus a wide variety of other random stuff from chronomancers to celtic historical setting classes. People were ready for a change, and 3E was just around the corner.
That said, 1E books are potentially cheaper (see amazon), and the artwork is more "eldritch". Monster Manual 2 is probably a high water mark for monster books for the game IMO, and Fiend Folio has some highlights even if it's more inconsistent (and was widely disparaged when released). White Plume Mountain is my favourite 1E module, but everyone has their own list. Unearthed Arcana has some amusing tongue-in-cheek fourth wall breaking cantrips.
Dungeon Magazine is probably the other "motherlode" resource to raid (the print version, not the 4E website). There are several lifetimes worth of adventure for the plunder in there alone.
So pick and choose. You may find wrt splat that less is more, and just stick to the RC to begin with.