Steel Dragons (Dragons of Faerun 140) have the best HD-to-CR ratio for animate dead, assuming you use the Zombie Dragon template from Draconomicon 197. If your GM lets you find a Wyrm of War (Dragons of Eberron 31) for animation, the dragon's extraordinary special attacks should stay due to Zombie Dragon, or, if needed, awaken undead (Spell Compendium 21).