Though the post is 3 weeks old, I'll give my two cents. I've got a True Necromancer (imported from 3.5 Libris Mortis) and I've had all sorts of fun with the different undead creations in the campaign. I agree with the Zombie Dragon for the win, and quite suprised mine still exists with all the combat I've used her for (26 HD Song dragon). It's definitely worth the trouble if you've bolstered it properly (ie; desecrate, etc.). I agree with Fleshgrinder about PF releasing their own version of "Libris Mortis", though adjusting the old book to PF rules didn't take too much work; still a bunch of the feats need to be updated. If your DM will let you take the "Corpsecrafter" feats from that book, then I would say Zombie Dragon whenever you can. Other 40 HD options would be Ogre or Giant skeletons, making them bloody or burning is always a plus. An old Dragon magazine (#324) trick had mentioned taking the corpse of a squirrel and using polymorph any object to turn it into the bones of a dragon turtle (you could get anything up to 20 HD really), and then animate dead...presto, Huge tank for the win!