Zara gets my vote. In my last emperor game, I was happy crushing everyone else with cavalries and infantries, and then with tanks. Then I realized that while i was out warmongering, Zara sat quietly on another continent, grabbed a nice 23% of the land and a good tech lead, with advanced flight! Since I had to take on him to win by domination (the only other remaining target has defense pact with Zara), and I did have a few mech infantries in service, I declared war on both AIs and expected to see my stack of tanks to come under heavy fire by Zara's stack of airships.
Never happened. I saw about 5 airships the rest of the game, and his biggest stack is no more than 15 units, mostly pre-rifle. I was really disappointed. I mean, to be fair, I didn't exactly crush him (didn't have to, i captured 3 of his cities, sued for peace, then crushed the other, more backward AI instead), but still, with any other AI, given the similar lead, my 20 tanks would be gone like that.