Barbarians. I am tempted to say Gauls purely because of forrester warbands but I'll have to go with Germans as they have more than one elite unit.
Civilized nations.. Definatly Parthia just because of the Perisan horse/Cata combo. For a siege tho I'd prefere Greece tho as Spartans is the only phalanx unit that can be used succesfully as shock troops as all of the Grecco and Eastern factions have a complete lack of shock infantry.
Romans.. Kinda the same but I'd have to go with Brutii as they have the biggest potential income.
The greatest happiness is to vanquish your enemies,
to chase them before you, to rob them of their wealth,
to see those dear to them bathed in tears,
to to your bosom their wives and daughters bring.
-GENGHIS KHAN