3 groups:
Nubia: the bonuses to ranged units and the brute power of the Pitati Archer UU just make it irresistible for competitive domination. The district building boost is also quite potent, so they have so versatility. A lot of MP players considered them to be very very strong.
Australia: they just get such a cheese ability with their production bonus, and the other boosts to district yield for appeal and the outback station make them a super economic civ. 100% bonus production for 10-20 turns is just overwhelming. And the outback station is game breaking if things go that long. In SP, it's probably the best UI, and I would rank it above the terrace farm if we just look at the improvements alone. They used to have the best raw production economy by a nice margin, although with GS, the Inca and Germany (coal plants) are up there too.
Aztecs: where nubia has early game power, the aztecs probably have the most all game power of basically any civ. The combat bonus + district rush is a big part of this: if you can expand to a second continent, you can have +8 CS from luxes. It just snowballs from there. I would argue the district rush ability is also top tier because it lets you transfer production at a better than 1:1 ratio between cities if you have liang, pyramids, or serfdom.
You can see the theme. Strong war ability + strong economic ability = extremely flexible power.