First, drop the idea of creating a unique name and stat block for every undead concept, as we see in AD&D (Fiend Folio, MM2) and later editions. All you need to know is:
- There are four basic forms of undead: corpses, skeletal corpses, corpse parts, and phantoms.
- The basic walking (and fighting) corpse is 1 HD; basic skeletal and partial corpses should be less, unless it has special abilities. Phantoms should probably get base 5 HD, but this can be lowered for less effective phantoms (no attack, easy to disperse.)
- Each special ability or defense adds a hit die; extra special abilities or defenses get +2 HD.
- Powerful, lordly undead are somewhere up in the 7 to 9 HD range.
You can pretty much make up undead stats on the fly, as long as you have a basic concept of what it looks like and should do. Flying skull that's hard to hit and paralyzes everyone in the room with a shriek? HD 4, AC 5, Move 18. Crawling torso that hugs your legs, preventing movement? HD 2, AC 9, Move 3. Crawling hand that hides, attacks by ambush, strangling the living? HD 4+1, AC 5, Move 6; damage 6+ means it grabs a victim's neck, doing auto damage each additional round until destroyed or removed. Skeletal snake that eats a hole in a sleeping human, crawls inside, and takes over his body every time he falls asleep from then on? HD 5, AC 7, Move 15,
Next, keep in mind the special bits common to all undead. They are fearless, except against clerics; no morale checks except for Turn Undead. Most are mindless, meaning they are undetectable by ESP and can't be reasoned with or even distracted by food, as even a "mindless" animal could be. If you have bleeding and crippling rules, or just critical hits, undead should be immune to them. Impaling weapons should be pretty much useless against skeletons and maybe even walking corpses.
And optionally, make undead even more resistant to normal attacks. They're already dead, right? It should be hard to kill them again. One possibility is not counting ordinary damage against hit points, as I toyed with here. Roll the creature's hit dice after every attack that it isn't completely invulnerable to, and if the damage from the attack is higher than the highest die rolled for the creature, it is knocked down, but not necessarily dead. Otherwise, the creature takes only 1 point of cumulative damage from any attack, except from attacks it is vulnerable to (smashing attacks for skeletal corpses, fire for ordinary corpses, silver or magic weapons for phantoms, plus other attack types as deemed necessary.)
Personalize each undead. Some may only be truly slain by destroying some object (like its head, or a magic amulet) or performing some other action (blessing an evil altar with holy water.) Some have a specific desire, giving adventurers an opportunity to bargain with the otherwise implacable enemy. Some curse their enemies when destroyed. Some charm their enemies and create armies of slaves.
The undead just seem like an endless opportunity for fun and horror.