I'm certainly for lethality rather than any of the others.

Even if what Louis is right it is better than the others. Red pretty much said it all. Especially better than Morale and Def ability.
Morale because it seems the penalties for large stacks are significant, but if we get down to individual units duking it out there will be a lot of fights where the loser will fight on until about 95% losses. Not cool.
Def ability of course only works forwards and to the right. So tweaking that will create odd battles where certain units will get crushed while other equal units win out easily.

Armour is my second choice. But since we are not certain how that impacts fatigue (if at all) it can't become my favourite.