Ghorid Madness!

Seriously, these guys rock.

Mostly, I suppose, because its insanely easy and cheap to recruit ghazi mercs in their territory. Some areas have 4-6 each, and the regenerate quickly.
Load up on those guys, pop into your neighbors city, build a ram, and just tell your ghazi's to 'advance'. Pretty much automatic victory against an equal or smaller sized force.

Plus, seriously, these guys just look awesome. With their dark-red faction color and their black-clothed elite infantry...well, you get it.

Having taken over India, central Asia and most of Iran, I frequently find myself thinking "THE THOUSAND NATIONS OF THE PERSIAN EMPIRE DESCEND UPON YOU" prior to battles. I've pretty much everybody except vikings in my armies at this point. Only in my version, the 300 get run over by mercenary elephants.