I like fighting against the Seleukids, because early in the game they will send out huge stacks of hellenic native spearmen, which may be the weakest unit in the game imo. This leads to tiny horse archer armies of like 300 killing 3000 seleukids or something, which is always fun.

I really like fighting most factions later in the game. Currently playing a Carthage campaign, I am trying to stall and conquer everything EXCEPT Rome so that they will hopefully build up and when we do finally start to battle they will have some serious units to throw at me.