So, I'm not going to break this down by edition or anything. I'm just going to present you with huge, sprawling lists instead of actually picking single "best" and "worst" items.
BEST: Unearthed Arcana (3.5), Pathfinder Unchained, Player's Option (AD&D 2e), Rules Cyclopedia (Classic). Books that have dramatically changed what was possible in D&D and which are still acting as major inspirations in everything I'm pretending to create.
Runners Up: Thri-Kreen of Athas, Complete Spacefarer's Handbook, Dreamscarred Press' Ultimate Psionics, Rogue Genius Games' Genius Guide to the Talented [CLASS]
For the worst... much shorter list:
WORST: Book of Exalted Deeds and Gamma World Player's Handbook (6e, d20M). I have never been as angry about any roleplaying product, ever, and they are literally the only roleplaying products I have ever regretted purchasing.
They are, in fact, so bad that I can't think of any runners up.