In a game of hypotheticals which is what Foe is and isn't, (let's face it with a new and retroactive scoring system every so often, there is no knowing.) Babel is the best population GB.
Without a level cap, without a future era cap, potentially limitless population in the smallest available population GB of it's kind. Mathematically at present, ya can't beat it.
Yes the FP cost of leveling up gets expanded exponentially. My point however, is that if time is not an obstical (let's say, hypothetically, you pass your city on to a grandkid as inheritance and so on it gets passed along, the Babel has no limits like the others only it's smaller.