I love it when there's a lot of magic, that's why I choose Archmage, and since Order is all about magic, they should automatically be Archmages for me:D.

Regarding the topic, I'll say that there are no classes with a more or less good class bonus for Order. I think Illusionist, Enchanter, or even Sorcerer would be неплохим (not bad), but not because of the class bonus, but because of Chaos magic itself.
Enchanter and Sorcerer are difficult to train due to the lack of corresponding guilds. Monk is very неплохим (not bad) when you mostly have to fight Chaos.
But I voted for Battle Mage because most often you need a mage who will be casting spells, and not using up all the resources every round IMHO.

P.S. Something's making you all obsessed with Archmages:smile35! Tell me, what's so special about such a hero? The fact that he knows a lot of spells - what does he need them for? He only has one turn per round in battle - he simply doesn't need that many. And besides, to level up a hero to be a full-fledged Archmage, you won't have enough skill points, and you also need him to survive in serious battles.
Now, let's do some math:
Level 18 is 74,800 experience, level 22 is 156,000 experience. Thus, to level up 3 heroes to level 18 with one purchased level, you will need (74800-12600)*3=186,000 experience, which is equivalent to one hero at level 23!!!