That military campaign quest is actually semi-broken right now due to using the wrong variable in the module system. They have a check that sees how far away you are from the quest giver, and if it's too far it isn't supposed to call you. The problem is that check is looking at your distance from the leader of the party instead of the actual party (which doesn't work), so it displays an error and gives you the mission no matter how far away you are.
My favorite troop for my armies are Nord infantry. I have them form a tight line and advance across the field toward the enemy. The third tier (Trained Footmen) make for great cheap combat units, which I use in garrisons for their inexpensive effectiveness. The Warrior level and above can really hold their own in most situations. Just training groups up to Warriors and leaving them at that is a relatively inexpensive fighting force. For the really tough battles (like a large siege or fighting off an enemy campaign of 1000+) I'll bring along the higher tier units.
Nord archers are surprisingly effective too for their price. They cap out weaker than the Vaegir marksmen but are better than the tier below marksmen and being lower leveled units are cheaper to train and upkeep than the Vaegir marksmen.
I may be biased though, I've played the Nords pretty much constantly since they first added them to the game