What are your essential tools for good gaming?
Dice and paper and pencil.
Seems simple doesn't it? That is all I really need, the essentials. I usually have a good idea of the targets numbers required for a game system and from that I can pretty much do anything. For monster stats (for which I usually use the stats as found in a book) I can forgo the book and just run a monster by "feel"; ie the monster dies when the PCs have gotten in a few good hits. I actually do run by the rules and written material but I can improv, even with the stats, when needed.
However, there are a bunch of other things I prefer to have either in game or as I prep. I prefer a DM's screen if only to hide my die rolls. A map of where the PCs are. The combat stats for the antagonists. Written out encounters. My reading glasses so I easily see the writing on the paper. A drink. The rule books within reach.
For prep I tend to get help online. Kobold Fight Club and Donjon help a lot with encounter building. I also have literally hundreds of rpg gaming books which I use as a research library. If I am home-brewing I will look at them to see how someone did something or to get an idea.
So, the essentials are very light but I do prefer to make use of all of my available tools.