Persia, because of its overall consistency. It is now overlooked thanks to Korea, Maya, and Poland which all have immediate bonuses (with Maya Pyramid and Korean UA first and foremost), but Persia is, to me, something more than a sum of its parts.
It has a great UA that both encourages and rewards being expansive (grabbing as many Luxuries as possible by rapid settling and faster conquest). Works great with Liberty, with its Worker speed, Settler speed, and the free Golden Age. Bonus movement for military units is fantastic (moving archers on hills and firing or moving siege engines, setting up and firing in the same turn), and it really helps with Workers too, making Persia great at creating infrastructure. However, going tall and getting resources from trade is also viable - and proves that Persia's UA is versatile.
The UU speaks for itself - stronger, and heals at double rate. The Immortals are even better when upgraded to pikes, and coupled with the Faith Healers pantheon belief. You can have a rapidly-replenishing army that can dominate late classical and medieval warfare, just until it obsoletes and no more conquests are necessary.
In Renaissance era, after all those city captures, the UB comes to play. Satrap's Court, with its bonus happiness will help mitigate the effects of going wide and conquering. Being a Bank, it will also be built in puppeted cities, meaning that the entire empire will contribute towards Golden Ages. More longer Golden Ages means more production (wonders), and more Social Policies - bonus culture will greatly diminish the extra cost of policies that is caused by settling many cities. But in turn, more cities equals more monuments and culture buildings, and with bonus production from GA's (and all the cities), Persia has a greater chance at winning the World's Fair and getting even more culture and a free SP.
Overall, it's one of my favourite, and one of the strongest to this day, vanilla civs. It favours a hybrid approach, being capable both at war and building up cities and infrastructure, and its bonuses are always useful, whether you're wide or tall.
Lots of dynasties indeed..
Hurry Production slightly in the Capital when you conquer a City, and a copy of a Unique Unit from a Civilization not in the game upon conquering an enemy 
Capital. Melee Land Units gain a +15% 
Combat Bonus when attacking for each point of 
Movement they are faster than the target.
