And few things are as delightfully terrifying as the Tzimisce.
For general gaming purposes, however, my most used and loved undead are juju zombies. Juju zombies are intelligent undead, meaning that in most games (read Pathfinder), they are able to retain class levels, giving them a broad range of powers and potential to throw at characters, and I like saying the word "juju." I hadn't seen them in anything but Pathfinder, actually, then discovered them when I purchased the AD&D Monstrous Manual recently tucked into the Zombie entry. I was thrilled, and look forward to using them quite a bit. I've used them already in my trap entry, and I've tossed them at players a few other times as well.
Forgotten Realms juju zombie realness, dahling.
My absolute favorite use of them, though, was as cohorts. Now, I've talked about my love of Clerics previously, and one of my favorite characters I've played was an Undead Lord by the name of E.B. Thurgood. This was a group DMed game, and everyone approved juju zombie as a variant template that I could use for my corpse companion. Benny, the Suli juju zombie barbarian was hands down the heaviest hitter that our low level party could muster, and his damage reduction and energy resistances meant that he was hard as hell to take down. My cleric kept him and his dead sister (an Oracle of Life) in the back of his cart and just stitched and reassembled them when they got hacked up, animating one or the other when needed. Both of them, in life, had signed contracts with him giving him possession of their bodies after death, and both resented it. It was a fun game.
But yeah, undead. Not my favorite. Apart from juju zombies, I really haven't used undead much in any games I've run.