Caelum

[Zurai]

Being able to fly three provinces on the strategic map with most of their troops is absolutely huge. It forces opponents to build defense in depth in their empires, which is incredibly costly and weakens their defense overall. Build a Storm General/15 Blizzard Warrior/10 Spire Horn Warrior force or two and go around capturing weakly held enemy provinces, setting taxes to 200, and immediately skipping off to another province. You'll do severe damage to their economy, not even to mention the morale damage to the player (if human).

While you're having fun in their backfield with the light raiding parties, build an army with about 5-10 Mammoths and double their number in Wingless, with supporting troops, and attack their front line. With the very high morale of the Wingless, your Mammoths won't break very often and will do incredible damage to anything but Giants.

Caelum has an incredible one-two punch capability just with their troops - this is ignoring their cheap and powerful 3A mages who can cast a number of nasty, evil rituals and battleground spells.