http://aow.triumph.net/forums/topic/hero-stack-abilities/
Leading is being defined as the hero occupying the first army slot, for easy reference.

Which hero will do that is dependent on a few factors:
1) Your leader will always lead.
2) Second, the hero with most leadership abilities(number of, not cost or level requirement)
3) When tied for party abilities, the hero with highest level will lead

After that it gets really muddy.

http://age-of-wonders-3.wikia.com/wiki/Abilities/Leadership

この辺りかな
おおむね>>350の内容と同じ