Best: A toss up between the Red Box set, which was my first D&D book ever, and the 3.5E Player Guide, which is my "home" edition. Both were great reads for very different reasons. Both were definitive books for my RPG life.
Worst: I'm going to go with Races of Destiny from 3.5e.
There are lots of D&D books that I dislike with a passion. I hated the lack or organization in 2e material. I hated how the Magic Item Compendium and Book of 9 Swords completely changed 3.5E. I hated stealth errata in Complete Psionic, and terrible spells like Melf's Unicorn Arrow in the PHBII. But all of those books had some actual ideas and some actual value, even if I personally dislike them.
Races of Destiny, OTOH, is without a doubt the least remarkable book in my RPG library. It came in a box set with the other Races Of series. I read it, I put in on a shelf, and it was done. It's not terrible; it doesn't break the game, it isn't annoying, it isn't offensive. But there's nothing useful in it. There are no new ideas, and none of it's concepts are flushed out. It is by far the most boring and worthless RPG book I own. I don't think WotC ever referenced any of the material in it ever again. The only thing remarkable about it is how unremarkable it is.
Worst: I'm going to go with Races of Destiny from 3.5e.
There are lots of D&D books that I dislike with a passion. I hated the lack or organization in 2e material. I hated how the Magic Item Compendium and Book of 9 Swords completely changed 3.5E. I hated stealth errata in Complete Psionic, and terrible spells like Melf's Unicorn Arrow in the PHBII. But all of those books had some actual ideas and some actual value, even if I personally dislike them.
Races of Destiny, OTOH, is without a doubt the least remarkable book in my RPG library. It came in a box set with the other Races Of series. I read it, I put in on a shelf, and it was done. It's not terrible; it doesn't break the game, it isn't annoying, it isn't offensive. But there's nothing useful in it. There are no new ideas, and none of it's concepts are flushed out. It is by far the most boring and worthless RPG book I own. I don't think WotC ever referenced any of the material in it ever again. The only thing remarkable about it is how unremarkable it is.