Unearthed Arcana is the winner
While the poll will be open, I am calling it.
Unearthed Arcana wins the core rules.
The top ten follows. First numbers is raw votes for, second is % that liked the product of those that expressed an opinion. You can see a pretty strong correlation.
Unearthed Arcana 275| 0.8899676
Spell Compendium 241| 0.8607143
Lords of Madness: ... 211| 0.8577236
Draconomicon 231| 0.8461538
Libris Mortis: ... 212| 0.8412698
Magic Item Compendium 214| 0.8136882
Fiend Folio 187| 0.7957447
Manual of the Planes 190| 0.7630522
Monster Manual III 178| 0.7385892
Expanded Psionics Handbook 186| 0.7294118
Dungeon Master’s Guide II, Oriental Adventures, and Monster Manual II also did well.
As for the worst, that would be Enemies and Allies, worst of the core rule and monster books.
The bottom ten, first number is dislikes, second % not liking of those that had an opinion. Less corelation here:
Enemies and Allies 131| 0.839744
Exemplars of Evil 74 | 0.72549
Weapons of Legacy 154 | 0.719626
Epic Level Handbook 189| 0.713208
Magic of Incarnum 132| 0.709677
Psionics Hanbook 152| 0.635983
Deities and Demigods 146| 0.621277
Miniatures Handbook 122| 0.589372
Elder Evils 55| 0.561224
Dragon Magic 92| 0.557576
Monster Manual IV, Book of Exalted Deeds, Tome of Magic, and the Arms and Equipment Guide also did poorly.
Thanks to all that voted. Round 2 to follow.