All of them. Sometimes just for inspiration, sometimes for material to convert (spells, monsters, magic items, etc), sometimes for material to use as is (maps, adventures with the edition specific material swapped out, lore), sometimes just for art (when the pcs encounter a monster, I use the best available picture- so if you encounter a grell, you see the 1e full page art from the Fiend Folio, if you encounter a manticore, you see the 3.5e MM art, etc).
I usually use old edition D&D books as my bathroom reading material, so I am pretty much always looking at something, and it's usually something related to or inspiring for my current games.