Which commander is the best?

They are all good! Necromancer with Soul Eater is the best, but that doesn't mean you should only take him. The Beast is good for dealing damage, while the Ogre Chieftain is better for blocking, which is more important. Just because of one instance where a level 1 Ogre commander stood and withstood attacks from 5-6 packs of 40 Troglodytes, I gave him the title of leader, and that's not even mentioning the Ballista!