I'm not going to rate them all, but here's my view of which are hardest from personal experience.

1. Pontus - hardest by a decent margin in my opinion
2. Hayasdan - Personally, I found this campaign much easier than Pontus, but still difficult.
3. Saka
4. Sauromatae
5. Pahlava

To be honest, part of the reason all three steppe factions are there is because I am terrible with horse archers, I just don't use them well (I'm far too impatient for them). The other reason is poor economy.

Other than those five, the rest I've tried haven't been so bad. Casse and Saba are actually pretty easy if you ask me. The start is daunting, I know, but once you're past that rough patch, you have plenty of time to expand at your leisure and take rebel provinces. Saba does have a hard mid game because you're going to be facing superior quality troops, but if it's getting too tough you can always resort to pantodapoi phalangitai.