Best combination coop

Civ combination does not really matter. Most of the time, it's better that each player attack on their own anyway. In quests when you're attacking together, you can decide who will be meatshield and who will use ranged. All civs have all types of units, even if it's not their best unit. So for regular play, it doesn't matter; if you're going for speed records, some combos will be faster than others for sure.