What is your favorite ultimate ability for that class?

Constant luck is great, but all other skills are also very useful, except for the Necropolis and Haven factions.

You're kidding, right? Have you seen the Necropolis's super ability yet? -6 to enemy morale, reduced initiative, reduced luck – with this super ability, you'll only be theoretically aware of true luck. The Haven faction isn't doing so badly either, with the right tactics. And you need to consider the path to achieving this particular skill, because for some, for example, the Elves, you have to take completely unnecessary skills.