Draconomicon is one of the best 3.5 books released to date. I felt that it hit the mark on so many levels (besides just the artwork): great "fluff" on dragons, good tactics for playing one, cool new and updated monsters, feats, items, and spells to really help beef up the mightiest of D&D's monsters.
Libris Mortis, while not terrible, was dissapointing. It had its strong points, like the diet-dependencies and the variant liches, vampires, skeletons, and zombies, but too much of it felt like something was missing. I was really hoping for feats or templates to bring some of the cool new undead abilities, like unholy toughness and unholy grace, to bring the MM monsters up to par with the newer undead, but none were to be found. I was also hoping for more template versions of existing undead. Someone stated that it is an essential book for necromancers, and for that I agree. And I think that's why I think it is the most disappointing of the bunch: I was expecting it to focus on the undead, not on those who ally with or oppose them.
Lords of Madness, on the other hand, far exceeded my anticipations. I love the chapters focused on several of the iconic aberrations, as well as a few interesting ones that aren't quite "classics" (yet). Each chapter reminded me of the old Ecologies in Dragon (back when they were useful), and having the feats and magic specific to the race within its chapter is very nice. I really hope they extend this format to future books in the series. (I've got my fingers crossed for a book of fiends, with the demons, devils, yugoloths, demondands, etc. each getting the full chapter treatment). Additionally, they updated some really great creatures from past editions (the hive mother, ulitharid, and urophion) as well as some more obscure gems (like the illithidae). The tsochari, although obviously inspired by Stargate, are still quite well done and stand up well next to the other iconic aberrations.