I use so much of both the 3.5 and 4E DMGs. I actually find 5E to be the least "fun" for me from a DMing stand point, but I love it from a player and homebrewer position.
From 3.5, I use the settlement design tables. I also use the entirety of the magic item pricing system, and am fiddling with how to make it for the scale of item rarity costs in 5E. How I use 3E items is weapons and armor have their enhancement bonuses halved, round up of no secondary enhancement and round down of it has something else (so a +1 sword stays +1, but a +1 flaming sword just becomes a flaming sword).
I also use the tables for making maps for different terrains and the encounter distance math.
In 4E, I use skill challenges (with my own modifications) and I use the encounter templates. The encounter templates give you ideas on how many of what kinds of creatures to use to get different encounters, and they're really good starting places for getting ideas.
3E had a ton of other DM supplements that excite my creativity. The Manual of the Planes and Deities and Demigods give me so many ideas, I still regularly read them cover to cover instead of just using them for reference. Heroes of Horror and Heroes of Battle are also great supplements for Horror and War games. The Book of Vile Darkness is a good (evil) book for inspiring evil.