Mongolia and it's not close IMO. They're the only civ I've had success with in offensive war on post-patch Deity (although I haven't played Babylon, who can push early Longswords with the Writing Great Scientist ).
On Deity, it is critical to have a very high kill/death ratio, and a fleet of Keshiks are excellent at this. If you play well, they'll never die. They accrue experience so fast that by the time they need to be upgraded, they'll have a full complement of open terrain combat bonuses to be used as Cavalry/Tanks/Modern Armours. If you stay roughly even in tech, having such supersoldiers will guarantee defensibility pretty much all game.
Another note on Keshiks - they are also excellent at keeping your other units alive, because of their movement (scouting) and swarming ability. This helps your melee and siege units end up in fewer compromised positions, keeping them alive long enough to become supersoldiers as well.
Finally, Mongolia's Keshik is very close to the Astronomy Renaissance beeline, so you don't have to go far out of your way to get them.