Power is ease of winning; overpowered is being able to unilaterally force a win condition with little counterplay. For warmongers this means having such an upper hand that taking out a neighbor empire (earlier the better) is practically a foregone conclusion, no matter how good the defending commander is. For others this means generating enough domestic yields that no matter how well your empire plays, they will squash you if you don't kill them immediately.
For example: warcarts are immune to spears; there's simply nothing you can do about them. Economic example: seowons simply existing.
IMO the two standouts are Hungary and Maori.
Hungary
Hungary's Raven King itself is the offender. It's less the power level of the ability - after all, several leaders can get +5str/+2 move on their whole army with some pretty flimsy CBs - but the timing.
If you were worried about Hungary rushing you
at some point in the game,
you can always play Germany and get that sweet +7 vs CS units to send him back to the childrens table. No, the problem is that they can rush people with boosted swords before the target can actually get swords out. Paying to levy isn't an all in strategy (you can still have a regular economy and army supporting you.) The free upgrades is where the problem is. If you both get iron working on the same turn, you'll still need time to stockpile iron and save to upgrade warriors to swords. Hungary can just snap their fingers. Swords vs +5 swords is challenging, but Teddy can do that. The problem is it will be warriors vs +5 swords and archers, which is a genocide campaign.
I admit I like the design of the Raven King ability. It has great flow. If Hungary still had to pay gold to upgrade those troops, this timing problem would be pretty much solved. If Hungary gets too delayed then any target has enough time to engage in counterplay, like building up their walls or just taking out the CS units before hungary can upgrade them.
Maori
These guys are just stupid. Ignoring the ocean start for a second, the marae's absurd levels of culture and faith and tourism for the cost of no writers is such a bargain it's insane. It works on floodplains.
Floodplains.
Now the normal way to deal with things like that is to just go to war with them and burn the marae down. But Ed thought about this and decided the Toa should exist, which is such an insane unit that the maori are the ones who can warmonger better. You can send
knights and toa can beat them +1 under oligarchy. Toa are super varu; they have the same combat stats and cost but superior promotions, oligarchy support, Pa building (because regular fort building was simply too casual) and they can be upgraded from warriors. It's just too much power for one civ. I know the counter argument is that they could die to a hurricane on turn 2 but they problem is, all the games where that doesn't happen and they actually settle a couple cities next to each other - it's over. In civ5 BNW, spain was often banned in MP because if they started near a NW, they could buy a free settler with the bonus gold, which was basically GG. A lottery civ. Maori is like that except the lottery is to see if pass the tiny chance to lose. Otherwise you get to go home in the new car with a million dollar check and a turkey in the oven.
Why I don't think the Inca are OP
The inca are strong, yes. But most of their oomf relies on good terrain rng and comes in the form of tons and tons of food. If any of their abilities gave production - like the trade route ability- then they would hands down be the best civ.
Food is good but it's just not science or culture or production. The warak'aq is also very good but it comes at machinery. If you start near inca you actually have time to counter warak rushes. (I think most players find it harder to use waraks than
@MarigoldRan does.)
Mali becomes very strong in the midgame but he's so vulnerable getting their with his -30% production, he is truly a boomer civ. I suspect suleiman has some sleeper OP potential but I haven't played ottomans yet to really work out how feasible a ibrahim serasker rush is.