I like most of them. Byzantines are good for organized, full fledged attacks. Teutons are good a defense. The English, Spanish, and maybe the Goths are good offence. The Vikings, Celts, and Chinese ooze havoc and are highly annoying. And the Franks are good at guerrilla war. But all this is in MY experience, and it might vary from person to person.
And it also depends on the time limit you want, the map, your allies, and who you are going up against. If you are Goth, and are spitting out swordsmen at an army of Cataphracts with 4 or 5 monk, guess who is going to win.
the Cataphracts