While I think Portugal gets the superlative, I'm surprised no one has mentioned the Byzantines yet. At least, I didn't see them.
Assuming you can get Crusade (their achilles weakness and why they lose to Portugal), they're insane.
They build a Holy Site, which gives extra Great Prophet points so they get an early religion, and can get Crusade. They build a Hippodrome, which gives them free Tagmas. They go to an enemy city, kill a few units with their Tagmas, spreading their religion due to TAXIS to that city and converting it. Their Tagmas now do full damage to the city walls thanks to PORPHYROGÉNNĒTOS, which combined with their +4 CS due to being next to other Tagmas and +10 due to Crusade, and +3 per Holy City following their religion, they get +17 attacks minimum (because their own Holy City counts)...meaning that they can take out that city pretty easily. They then have a new city, with the potential to add four more Tagmas for free that have no resource requirements. That's enough to take on two more cities at time...
Even when Tagmas come to an end, it's only a loss of +4, and they should have Holy Cities converted to make up for that.
It really is a snowball civ. It just takes a while to get going (your first districts have to be Holy Sites, then you want to get your Hippodromes (Entertainement Complexes) researched and built in order to get going...but once you do, the everyone is in for a world of hurt.