Warrior, Mage, Rogue.

I would say that all three classes are equally viable. I choose based on my mood or depending on the map in the second Heroes game.

In the third Heroes game, I either choose a Mage or a Warrior. A Mage in 80% of cases, because in the third game, you can take any hero, and magic bonuses are great. It's very difficult to level up a Ruler through battles.