. hmm interesting question IliaDN
i have to say taking into account not just the army but the whole faction eg location shrines etc i have to say thrace ,
they have boring units although a strong cavalry, and again boring battle tactics ,
in my experiance always envolvolving onagers from range fring at you and wating for you to come to them, and they also are bordered by weak factions eg dacia and scythia making the campaign to easy at the start.

although i would agree Gaul come a close second