A Warrior is probably the most powerful within Reach. An Assassin (Theives have no offense besides Pickpocket Poison) is the most powerful if they see you first. A Mage/Archer (Artillery, the third leg of the combat tripod) are not as powerful but have Range Advantage, neither of the other two can hurt them without closing. Between the two, Archer tends to beat Mage because they have better Range, Armor, and Damage. Mages have higher RoF, more effects than straight out damage, and AoE.
It really just depends on the fight, and how you play. In a melee, or duel, the Warrior is generally best with the Armor to survive, and powerful weapons to finish them quickly. Against an Army, the Mage can hit the most at once, set them against each other, summon powerful allies, and raise the dead as they fall (But probably take an arrow, or two before dying.) For a single target without a fight, and Assassin can kill just about anything in 1 Hit. Against a Dragon, an Archer is generally your best bet.
And that's if you're playing a pure Archetype. There's no reason why a Warrior can't be good with a bow, a Wizard wear enough Armor to take the Glass out of the Cannon, or an Assassin use Magic. So, the most powerful is probably a Spellsword, or Battlemage, because they can beat any point on the Rochambeau by not playing their game.