I played 1e in the 80s and then 5e starting in 2014, and no D&D in between, so I've missed a lot of D&D content. But my favorite D&D book of all time is Volo's Guide to Monsters. I know I'm in the minority and that was one the first books to get "legacy" status on D&D Beyond. I'm glad I bought the physical book (with the excellent Hydro74 Illithid alt cover). I love the combination of lore, lairs, and stats. I know many (most?) D&D consumers prefer the more statblock dense approach, but I've built entire story arcs from the inspiration I've gotten from Volo's.
While I like the dragon and giant focused books, Volo's was the right mix of depth and diversity for me.