Everyone, I believe I found one of the strongest low HD undead to have join your undead army.
The Time Wight!
It's from the old Monster Mayhem articles on the Wizards site, and it has a couple of fun abilities...
-Haste and Slow 3/day each at CL 5
-1/week 35% chance to summon another Time Wight for 1 hour; summoned wight is allied to the summoner, and can't use his own summon ability.
-Constitution Drain and Spawn Making; Fort save 15 or lose 1d4 constitution.
Why are they so good for the low level Undead master?
They are a 5-HD undead that can haste themselves and slow your enemies, giving them advantage; they have a 7 out of 20 chance to summon another one, giving you even more of them for free, and the summoned ones still have the Constitution Drain, Spawn Making, and spell-likes!
Another fun creature is the Gravbeast, because who can say no to a creature that gets 6 claw attacks on a full attack, all of which have their damage doubled, free tripping as a Huge sized creature (they are large sized, by the way) whenever they hit with a claw attack on a pounce as a free action? Or how about the ability to cast a weaker version of Reverse Gravity every 2d4+2 rounds? Or the fact that it can give itself a 50% miss chance against all non-ray ranged attacks for 6 rounds? And all the other nifty abilities that make this creature an awesome special mount for a Paladin of Tyranny.
Then there are the Transmutation spells from the Pets of Any Stripe Random Encounters articles; the first, Mind of a Beast, is a 4th level spell that replaces the mind of the target with that of an animal. You can apply Permanency; even funnier is its 7th level version, Mind of Another, which allows you to replace the mind of a creature with the mind of any of the following: aberration, animal, beast, fey, giant, humanoid, magical beast, monstrous humanoid, or vermin. It can also be permanencied, and the prank potential is endless; I mean, is there anything funnier/more cruel than replacing the mind of the Paladin with that of the Tarrasque?