Hi and welcome
I'd say the easiest faction and scenario is the Vespasians in the 4 Emperors. You need to be a wee bit careful to make sure you squash the Jewish revolt but its probably the simplest faction - the other two will fight each other.
its good to learn how NM, VPs, EPs are collected and used (you need to bribe the Danube legions) and the early campaign in Judea will help you get your head around sieges etc.
I've not played it under 1.3, but Sulla in the Marius-Sulla-Mithradates campaign was another side that was a bit easier (even if you start surrounded ... or in the central position if you are an optimist).
edit - you may find this AAR
helpful. It was with the beta so some things are different now (the legions have been toned down), but it may help give you a feel for the options and things to watch out for.