My favorite two books I own are Volo and Tome of Foes. I love the lore, found it a blast to read, especially the demon and gith bits.
The worst 5e book I own, which I regret buying and would return if I could, is Monsters of the Multiverse. It utterly neutered generations of lore, and for what? To supposedly eke out a few more sales from a younger generation? To reset expectations for an upcoming edition change? This isn’t even an anti-woke screed on my part, the material concerning the races/lineages is half baked, poorly thought and reasoned out, and just frankly embarrassing in places. Thank the gods for bg3 which put Dueger and Gith back in their rightful places and tones. BG3 proves that DnD doesn’t need a PG era to sell massive amounts and make tons of money.