Early: Agartha. I really like the nation, and the national units, even if I'm starting to accept that they suck.
At least when you don't consider magic, and before you get Darkness.
Middle: tie between Ulm and Bandar Log. I voted Bandar because I hope MA Ulm is pretty well represented already. I like Ulm's faith in strength of steel, and I kind of link the MA Ulm's siege engineers with EA Arcos' engineering culture. Black Priests aren't quite Philosophers, though.
Late: Man wins, with Atlantis second. It has mage-spy-engineers, which is awesome, shielded, armored crossbowmen, which is awesome, longbows and knights and wardens, and it even has a good chance of making Gargoyles. Gift of Reasoning them is hard, though. Atlantis is just cool. Inuit necromancers, clay golems, sailing amphibians, all kinds of cool stuff. Also, FINALLY ice armor on beings who can actually fight in melee.