The 1E DMG is, of course, at the top of the list. Still awesome and has seen use through all editions I've played. It was the first AD&D book I purchased and is in pretty sad shape -- held together by a 3-ring binder and those little disk things that reinforce the holes I punched in it after the binding broke. I've also referenced my 1E PHB and UA, but not as much as the DMG.
Lots of 3E Eberron stuff. I sold off some of my stuff after hanging up 3E. I ended up rebuying that and more when 5E came out. A lot of it is now digital, but I still feel dumb. I think I actually spend more time in the Eberron books than I do in the 5E core books.
I still occasionally reference http:\\d20srd.org for a variety of things. A lot of this has been to help convert some Eberron stats, especially getting a baseline for price lists. I have grabbed a couple of crunchy bits, though.
Dragon and Dungeon magazines running from the mid-1980s to the just into the digital age. These aren't referenced frequently, but I have pulled them down several times since picking up 5E and can honestly say they still earn their space on my shelves (they share a full shelf of a standard width bookshelf with my 1E DMG and about a ream of hand-written notes from my AD&D home brew setting).
Otherwise, most of what I care about from previous editions is minimal, but still in my head as far as inspiration requires.
Books I wish I still had and think I'd use would be the 3E Tome of Magic, Magic Item Compendium, and (maybe) Spell Compendium. I see the 5E Warlock as being a blend of the 3E Warlock and Binder and would love to do a bit more with vestiges. The shadow caster was also kind of neat and could be an interesting Wizard or Sorcerer kit.