I have to go with popular opinion, and say the Seleucids. Such a great choice of units. You can go for pikes with cavalry support, train legionaries if you want more of a close range fight, or you can just go crazy and bulldoze the enemy with Elephants and Cataphracts.

I really like the Germans though for their aggressive axemen. They're a bit more vulnerable than pikemen and legionaries so you're never quite sure if your line is going to hold which makes things more exciting/interesting. It's just a shame they're stuffed when it comes to assaulting large and epic stone walls.