Going to assume the map is something that isn't island plates.
With R&F... Korea is just so good because of the Seowon in conjunction with
the mines and farms boost it gives. +1 food from farms, guys. In the ancient era. WHAT. So it's really just how well you can exploit that science lead. I also think Zulu is blisteringly OP the moment they get the Corps civic, cause you can turn military power into anything if you have neighbors. But anyways...
Rome is very well rounded. Great infrastructure boosts and the best unique swordsman, which itself is the probably the highest impact unit in most games.
That said, Aussie has 3 huge advantages: production from getting dow'd, since you will get dow'd; boosted districts from appeal, very good but not reliable; outback stations. I would wager Outbacks are the best UI in the game. But they do need some swaths of flat space to really get things going.
Aztec district rush is better than Rome's bonus for infrastructure, and they have those crazy boosts to amenities and combat. Talk about having it all. You do need a lot of luxuries in your turf though, which means you need to get a foothold on other continents. Not as easy with loyalty unless you start near a boundary. So they lose generalist points there.
I think Germany is best in ICS though, because if you use the Hansa well, and I mean really exploit those suckers
, and you can absolutely explode in production over everyone. Truly efficient Germans can get a Hansa up to 30 production with the right layout. Hansa+CH produces independent of terrain, so you don't even need hills; you can monopolize the engineers and merchants with all those IZ/CHs you have; and in R&F, you can leverage several government and legacy bonuses better than anyone to fuel your district-based rampage. Classical Rep, Merchant Rep, & Democracy (this last one makes your carpet of districts absurd.) Plus you get the extra military policy from Fred the Red, so you can keep up a military production card + limitanei or retainers in those governments too. Not that you even needed the extra district slot, but it guarantees even a size 1 city can get it's CH+Hansa laid down turn 1 (for locking in cost.)
The 4 civs I mentioned are all very strong generally because production is power; but I'd cast my vote for Germany having the "most general power" because it's truly terrain independent and can out-produce pretty much everyone.