What is your favorite ultimate ability for that class?

I don't think a mage with Combat Alchemy (CA) can easily handle an equally leveled, well-developed hero (possibly also with CA) of another faction. There are several possibilities here. As I've said many times, the enemy might attack slightly earlier. Then, a mage with underdeveloped CA will have very few chances.

And Ora is my favorite mage. Especially in a long game. With Empathy and her native schools of magic, she can defeat any other mage with CA (IMHO). However, getting Empathy without a mentor is not much easier than getting CA. But it's safe.