Here, we're only talking about the main hero. For secondary heroes, I take skills related to estates, literacy, and artillery. Water spells, when earth and air magic are available, are usually left unused, so I don't take them. As for resistance... there are more important skills, and there's no room for it. It makes sense to take it if there's a chance to acquire artifacts that grant magic resistance.
Tactics is certainly a debatable skill. Sometimes it helps a lot, and sometimes it's useless in every battle on the map.
What makes ballistics particularly good for Barbarians? To charge into a town with behemoths?