3.5 edition had some great supplementary books. There were loads of them. One of the most interesting books though, was the Book of Vile Darkness, containing the rules for all things evil – cursed items, poisons, and truly evil organizations and monsters. Actually, in fairness it was a 3rd edition supplement, but whatever. In the monster section there was a new template for undead which I felt made for far more interesting undead encounters than the standard skeleton/zombie combination: the Bone template.
The Bone template (if I remember which one it was, I may have the name wrong) was the way to create skeletons and zombies which were more potent than an animated pile of dead parts. A Bone skeleton had been infused with its old soul, and while still under the control of the necromancer, retained its skills and abilities of its previous life. At last, there are skeleton mages, skeleton rogues, and skeleton druids. I never got the opportunity to use them in a campaign, and I no longer have the book, but they suddenly mean that an undead encounter can involve things the party would never expect. A quick and nimble skeletal assassin which leaves no trace of blood or flesh because there are none to leave; a barbarian zombie which uses flails made of corpses; a powerful lich with a cabal of skeletal mages to support it; all these things are now possible. Bone undead make by far the most interesting challenge for any party, and I wish they were used more often.