1. I think stunning is more useful than charging; peasants don't deal enough damage to rely on a second attack.
2. Archers.
3. It depends on the situation, but usually armored units: they provide excellent cover for their allies, which is useful not only in the endgame against an enemy with strong archers, but also when clearing certain neutral creatures.
4. Regarding combat griffons: the enemy doesn't often line up nicely enough to get a good charge attack off. You can often only attack two, at most three, creatures at a time, and even then, you're sacrificing the fact that you're not attacking the ones you'd ideally like to attack right now. And the damage from the combat griffons' ability is only half! But royal griffons, although they hit only one target, deal double damage. And their stats are slightly better. So, in all situations, I choose them, and I also think they are overpowered.)
5. Also, adepts, because of their cleansing ability.
6. Usually, it's the Knights of Isabelle. Although, when clearing certain neutral creatures, I find the paladin-angel combination more useful, but in the endgame, champions are definitely better.
7. In a prolonged battle, seraphs might be okay, as divine retribution is not a bad ability. But you don't know in advance if you'll be able to use it effectively, and resurrection will definitely be useful in the endgame, and when clearing creatures, so archangels.