Altar. They are quite frankly, the most powerful by a very long way, and also the most enjoyable imo.
You haven't tasted power until you've roamed the world with an Epic army of seven heroes, all of them >lvl 30. Make your soverign a mage (since henchmen have only limited magical capability) and use your army of henchmen as carriers for all the legendary weapons you inevitably pick up while adventuring. Watch entire empires fall to your little handful of people.
Forget about the other "magical" nations. Altar can have all the mages they want, mages with insanely high levels and tons of inherent defense without having to touch armor. Bonus points if you happen to make friends with pariden and buy a few spellbooks from them.
The free prestige from completing quests also allows them to reach incredible city sizes that take most people forever. And to top it off, they get a unique leather cuirass (early armor tends to be with you for a long time) and two powerful unique swords. As i've discussed elsewhere, swords are overpowered weapons, and athican swords are just that, with a cherry on top.
I like pariden too, but i don't really see how anyone can compare to the altarians.