I want to say cleric, because it has some nice defensive abilities like Gleaming Shield and Aura of Resistance as well as being able to wear Platemail and can use all healing spells. Not only that it has some abilities to protect your allies like Protectorate and Defend Other. Now I know other classes as some nice masterables and I have been working towards them as well, the purpose of this thread, was trying to get a second opinion on what class would be the best a main for a tank?