Alemanni is probably easy if you are aggressive early on and take risks, but when I played them on H/H, I just found myself in a long stale-mate with the Romans and it was years before I broke the deadlock and could take the town south of the Alemanni capital, and several years before taking the one further south. Every move to take a settlement was countered for such a long time. Once I had 3 settlements, I then ended up being the buffer between the hordes sacking most of western Europe and also the resilent WRE. Progress was incredibly slow. At times it reminded me of the early part of the original RTW Numidian campaign, but not quite as desperate.

As to most challenging faction, it is probably those Berbers when their infantry is rubbish compared to the other factions.