For me, it's the Forgotten Realms 1e gray box. I got the set not long after it was published in the 80's. Up until that point, I used the Known World, as it was presented in Isle of Dread and Red Arrow, Black Shield.
The FR box set was a real eye-opener for me. It included lots of cool information like mercenary companies and the Zhentarim, and had a real "Old West with swords" feel to it. This was before the 500 novels and supplements were published. I played for several years using just this set with my own home additions. My friends and I had a blast.
I really liked the wide open maps, with many lawless and uncivilized areas. Our game was centered at Boareskyr Bridge, which is a merchant tent city with only a couple mid-level adventurers keeping people in line. My friends eventually were the big movers and shakers in the area, and had to fight demons, orcs, and Zhent assassins. They also did a short recon of Evereska, from which they quickly retreated.
Our game maintained the stylings of the Old West as much as possible. There was a sword duel in main street at noon, orc raiders scalping human settlers, and other fun stuff. I really wish FR had maintained this. Instead, we got the Time of Troubles, Wild Magic, dinosaur people, Harpers under every rock, and the Chosen of Mystra.
I used to have a LOT of FR products. These days, I have the gray box and FR1 through FR6, and a fair collection of novels. I sold everything else.
I will say I like the new Waterdeep novels, even though they are set in the 4th edition timeline, since they are one and done, and generally concentrate on a few not-too-powerful individuals getting caught up in adventures in the City.