Undead who keep/lose class levels

[Ettina]

Which intelligent undead keep at least some class levels, and which ones completely wipe the slate clean?

[frogglesmash]

In general template undead keep class levels, while non template undead so not.

[OldTrees1]

Which intelligent undead keep at least some class levels, and which ones completely wipe the slate clean?

Too many to list and too many to list respectively:

1) Is it a template? (Yes: Go to 2, No: Go to A)
2) Does it make you mindless or otherwise state you lose your abilities? (Yes: Go to A, No: Go to B)

A) You lose everything
B) You keep stuff

Example: Wight
The Wight creature in the monster manual is not a template so it loses all its class levels.
The Wight template in Savage Species is a template. They are not mindless and do not say they lose their class levels. Therefore they keep all their levels.

[The Viscount]

Any intelligent undead that is a template, such as vampire, will preserve the original class levels they had before. If you become an intelligent undead that is presented as a monster, such as ghoul, you lose any class levels you had before. Any undead can gain class levels after the fact, but they'd have to earn them the regular way. An undead who has lost their class levels could theoretically gain them back via levels in emancipated spawn, but it's not advisable for PCs due to the ECL hike.

[Thurbane]

In general template undead keep class levels, while non template undead so not.

^ This.

Lich, Necropolitan, Gravetouched Ghoul, Sepuchral Thief, Vampire, Bone Creature, Corpse Creature, Death Knight, Ghost etc.

AFAIK, any undead template which doesn't set Int to a non-ability should allow you to retain class levels. There may be exceptions I am forgetting

[Zanos]

In general template undead keep class levels, while non template undead so not.
That's a bad general rule, since the two most common and core undead templates, skeleton and zombie, violate the generality.

[frogglesmash]

That's a bad general rule, since the two most common and core undead templates, skeleton and zombie, violate the generality.

The accuracy of my "rule" is determined by how many undead types do or do not conform, and not by how common any of those undead types are.

[OldTrees1]

The accuracy of my "rule" is determined by how many undead types do or do not conform, and not by how common any of those undead types are.

The quality of a rule is found in its utility.

A common measure of utility for a guideline would be mean time to failure (MTTF). Aka how many times on average does it take between users of the rule ending up with wrong answers (through user error, system error, or specification error).

So while your rule is accurate for most undead, its quality can be improved upon.

[Necroticplague]

Note that the OP specified intelligent undead, so mindless undead don't need to be considered. Under this narrowed space of possibility, the more complex rule, and the simpler rule, produce identical results.

[The Viscount]

Yes. If you need to make it a clear rule it's just: For mindless undead, they always lose class features. For intelligent undead, templates keep and monsters don't.