Hmmm, let's see, sticking to civs that have already been in the game, and keeping in mind that this is my best guess based on what I know of them currently, and am ammenable to reeevaluating some...
From Civ VI:
America: S-tier
Arabia: S-tier
Australia: Borderline A/B tier. Technically dominant in their region, but also alone on their island so...I lean B.
Aztec: A-tier
Babylonian: A-tier
Brazil: A-tier, much as I hate outting a colonial there. But they are a dominant regional power in SA and have been for a while.
Byzantine: A-tier. Might be a case for S, but I think half the Roman Empire should be a tier below the whole Roman Empire
Canada: B tier. Sure, striking enough, but dominant? In the shadow of America? Not so much.
China: S-tier, and the S-tierest of of S-tiers too. They've been a global or supraregional power for very large chunks of the last two milleniums. Despite the dark ages and the hard times, they keep getting back there, and would you look at it, here they are again. What more can be said?
Cree: Borderline A/B tier. A dominant player of the northern plains, but not quite with the impact on colonial powers that, say, the Iroquois had as kingmakers, so I'm reluctant to give them the full A tier (there really aren't many of those in North America in my opinion.
Dutch: S-tier (lower S-tier, but S-tier) due to their colonial empire, would be more like B-tier if they had only their European holdings to rely on.
Egyptian: A-tier. Never quite had supraregional or global influence, but were among the most fabled of regional powers in world history.
English: S-tier. We all knew that.
Ethiopian: A-tier. A long-standing major player in African history.
French: S-tier. Also all knew that.
Gaul: B-tier. I'm sorry, they're cool, I love Asterix and everything, but I cannot rank them higher just because they razed Rome that one time. I'd argue that there are better choices for "Celtic" representation, too.
Georgian: B-tier and low end B tier at that, not too far above C. Let's face it: they're a meme civ.
German: S-tier.
Gran Colombian: B-tier. Too short-lived to be any higher.
Greek: A-tier as Greece. S-tier as Greecedon. Greece without the Macedonian Empire just doesn't rise to the level of supraregional or global power; its influence never really extended past the eastern half of the Mediteranean. Get Alex here, and of course they immediately jump to S-tier.
Hungarian: A/B tier. I don't tend to view them as quite rising to the level of a dominant power in Europe, personally, so B, but I am not too versed into the history of this part of Europe so could be somewhat wrong. Definitely a major regional power either way.
Incan: A-tier, easily so.
Indian: S-tier as the whole of India. If split into various periods of Indian history, more likely a number of A, B, C, D and E tier states.
Indonesian (and proto-Indonesian kingdoms): A-tier
Japan: S/A tier. A for most of history, but arguably rose to the level of a global power in the first half of the twentieth century...with disastrous consequences.
Khmer: A-tier. A dominant state that exerted influence across much of South-East Asia.
Kongolese: A-tier. By far the most significant kingdom and state between the mouth of the Niger and the Cape.
Korean: A/B-tier. By technical wording should be a B, but that seems unfair to them as the only reasons they never really rose to a dominant role is they were stuck next to the S-tierest of S-tier civilizations ; it's hard to be dominant anything in China's shadow. That they stood out as much as they did in that shadow probably bump them to A-tier in my mind.
Macedonian: S-tier, but should be S-tier as Greece, since their greatest accomplishment is ushering an hellenistic age across much of the Ancient World.
Malian: A-tier. A dominant empire of the Sahel, whose economic fortitude had impact as far away as Europe.
Maori: AB-tier, for largely the same reason as Australia. Technically dominant in their region, but their region is also an isolated pair of Islands, and may be more known for their striking culture and resistance than for their impact on regional history.
Mapuche: A-tier, surprisingly. The civilization that halted Incan southern expansion and greatly delayed the Spanish one afterward can definitely be said to have played a dominant historical role. You could maybe make a case for B, but I lean toward the A - they're just obscure and very, very, very badly designed.
Mayan: A-tier and without much question.
Mongolian: S-tier. It's easy to view them as barbarians, but they were, in fact, a global power for much of the late medieval era. You don't stretch from China to Poland without getting that S-tier.
Norwegian: AB tier as a Viking representative: the Viking's status between a dominant power and one that wasn't actually THAT dominant, but highly striking is a little hard to place. They're both easy to overrate and underrate. (If this was the actual modern nation of Norway, they'd be C-tier though). No, colonizing a little corner of Newfoundland does not move them to S-tier.
Nubian: A-tier. Deep impact on Egypt and the upper Nile area.
Ottoman: S/A-tier. They easily qualify for the A, but were they ever really global, considering the Byzantine (wiith similar territory) got an A? I tend to lean yes, because of their role as the Great Boogeyman of European history (something Byzantine never had; if you have a whole continent shaking in fear of you, you might be S-tier).
Persian: S-tier, easily.
Phoenician: A-tier, both as themselves and Carthage, because the entire network of Phoenician cities and colonies never was really united - and Carthage, the most visible of them, was more a regionally dominant power than a global one.
Polish: A-tier, especially once the PLC is taken into account.
Portuguese: S-tier like the Dutch.
Roman: Very easy S-tier
Russian: S-tier also.
Scottish: B-tier. They're striking and cool, but they rarely if ever were the dominant power in the British Isles, let alone further than that.
Scythian: AB tier. They were certainly dominant on the steppes, but their historical impact on their neighbors is more brief, striking intervention than lasting dominance over other civs. Well, unless one count the Parthian Empire as Scythian and not Persian, but I would not agree with that. Hard to place quite properly. Lack of unity also hurts them.
Spanish: S tier.
Sumerian: A tier.
Swedish: A tier thanks to a certain Captain Gars, would probably slip to a B without his achievements.
Vietnamese: AB tier, same as Korea. Technically a B because of China's long shadow, but standing out in that shadow is arguably A-tier in itself, and I would give them the A.
Zulu: B-tier. Look...they were a bright explosion, but in terms of lasting influence, not so much. Iconic and striking, though.
Older Civs:
Assyrian: A-tier
Austrian: A-tier,only if Germany doesn't cover the Holy Roman Empire, because Austria's history is too intertwined in that.
Carthaginian: A-tier, see Phoenicians above.
Celtic: F-tier. They're a hodgepodge of completely different groups that had limited or no link between them; as a single civilization, that makes them largely a myth.
Danish: See Norway, above, since they're both in as essentially Viking representatives. As post-viking era Denmark, AB - a little below Sweden but not too far.
Hunnic: B-tier for largely the same reason as the Zulu. Striking, iconic, but not really a long-term dominant power.
Iroquois: A-tier. Dominated the Great Lakes region for a century, and largely served as colonial kingmaker.
Moroccan: A-tier. Berbers would essentially be the same for the same reasons.
Shoshone: B-tier (and so would the Pueblo whom they replaced). They're fascinating, interesting group, but their long-term influence over their region is...just not quite there (there really aren't that many A-tier in North America in my opinion). Move to A-tier if we count the Comanche as Shoshone (but we shouldn't)
Siamese: A-tier.
Songhai: A-tier.
Venetian: A-tier. Much as I don't like it.
Holy Roman Empire: A-tier. As Germany they rise to an S, but without they remain a dominant regional power, not a global one. As the Habsburg, they also rise to an S, but that's on the back of Spain, not the Empire.
Native American: F-tier. THAT was never a unified civilization and has no place in this game. Bad joke, Firaxis.