Warrior, Mage, Rogue.

A mage is more suitable for campaigns and for those who enjoy long games.

That's even more of an ideal option for me. I won't calm down until I clear the entire map. And it's easier to throw fireballs at enemies than to chase them down with other heroes.
By the way, my main hero in the second game is ALWAYS a mage. And for variety, I take archers and melee fighters as secondary heroes. I usually ignore thieves altogether. I don't even always build a guild. Raw power is my thing.