IMHO, the Halberdier. Even the unupgraded one has 10 hit points, which is a definite plus. It holds up well in defense, and if my memory serves me right, the Champions' cavalry bonus doesn't even affect it. Of course, this isn't a particularly important reason, but it's still worth mentioning. :) In second place, the Captain of Centaurs. Mainly because of its speed and high hit points. True, it's not as powerful in combat as the Halberdiers, but it's swift and brave in attack (running around the battlefield naked isn't exactly a walk in the park).