The Byzantines for me.

The unique unit roster is one of the Byzantines best features, I disagree with rvg on Byzantine infantry late on. Its true they lose the edge they had in the early period, but they still form the backbone of Byzantine armies throughout, mainly because there is no unit to replace them.

I enjoy playing the Byzantines in the high and late periods as its a big challenge, takes alot of skill to avoid bankruptcy and reconquer the empires lost territories.

The Byzantines have the greatest potential to expand enormously early on, you can overrun the Turks and gain access to the vast wealth of the Levant. With Antioch, Tripoli, Palestine and later Egypt your economy skyrockets.

You can become effectively invincable, with the likes of the Varangian guard, Kataphraktoi and high star generals commanding them almost nothing can stop you.