Other Good Targets:
Kelvezu Demon (MM2), (Ex) abilities: poison, uncanny dodge, evasion, +8d6 sneak attack
Jarilith Demon (MM2), (Ex) abilities: pounce, rake, augmented critical, improved grab, frightful presence, scent
Arrow Demon (MM3), (Ex) abilities: symmetrical archery, close combat shot, oversized weapons (bows)
For all of these, I'd prefer skeletons, but these guys do not have 20 HD as standard versions.
The BoED has examples where more advanced versions can be summoned with the planar binding/ally spells (in the celestial paragons section).
Assuming an infinite multiverse, there could be infinite Kelvezu out there... allowing for infinite guys that have advanced to 20 HD? YMMV...
Also, for the base skeleton, I'd prefer those made using the spell Vile Death for maximum HP per HD
(along with all of the other desecrated shrine, corpsecrafter and UA necromancer variant goodies... ref: K's handbook)