I don't know about the others, but the Sassanids are easy as hell especially with the Clibinarii Immortals/Cataphracts and the income that soars into orbit as soon as you take Alexandria.

I've only played proper campaigns as the Vandals, Huns and Sassanids so far (they are very fun and addictive to play as, I have played 3 full length hun campaigns already). Out of them the Vandals and Sassanids are extremely easy, but I had I little more difficulty with the huns. I'm guessing Berbers or Celts for the hardest, due to their rubbish starting situations.