What is your favorite ultimate ability for that class?

In my opinion, the best one is the Elven skill tree. By the way, don't forget that the path to it lies through the "Elven Luck" skill, which gives an additional +25% damage to attacks, although the path to it... isn't exactly easy. The Troll skill tree is also good, although Chaos can be developed much more effectively. The Dwarven skill tree is also good, in every sense. The Barbarian, Necromancer, and Inferno skill trees are also good. But, in my opinion, it's not worth developing the Mage skill tree; there's a high risk, and... you'll just end up stressing yourself out. The Knight skill tree is also good, even very good; you just need to use it skillfully.