For me, it's a toss-up between the Arms & Equipment Guide, Deities & Demigods, and the Book of Vile Darkness. I don't think I'll ever get any use of of these three.
... except for the NIPPLE CLAMPS OF EXQUISITE PAIN! Truly, these are the iconic vile items, and I will pimp the NIPPLE CLAMPS OF EXQUISITE PAIN every chance I get. There is nothing else that sums up so beautifully -- so EXQUISITELY, even -- the essence of vileness that Monte and friends have distilled for us. No campaign could ever be complete without allowing the NIPPLE CLAMPS OF EXQUISITE PAIN to make an appearance. Put these in your game, and I guarantee that your players will be thinking, "here's a guy who knows what a dungeon is REALLY all about!"
Hong "will wear NIPPLE CLAMPS OF EXQUISITE PAIN for food" Ooi