I have 100+ D&D books from all editions in hardcover and a few dozen more in PDF. Not to mention a few dozen Pathfinder products.
Specific to 5E, I have most of the hardcovers, plus D&D Beyond. I've not purchased most of the recent setting guides -- Eberron, Wildemount, Richtoffen's, and all the MtG stuff. In summary, I've got a lot of sh*t!
So what have I used?
Well, I've run about 200 games of 5E spanning 3 campaigns. I've used material from the Starter Set, the Essentials Kit, Tyranny of Dragons, Storm King's Thunder, Dragon Heist, Descent Into Avernus, Curse of Strahd, Tomb of Annihilation, and Princes of the Apocalypse. In some cases I've run large parts of the storyline. In other instances I've pilfered and repurposed a few maps, grabbed magic items or monsters, etc.
And all the splats and monster books have gotten use as well.