So much is situational.
America? Founding Fathers might be great. But if legacy bonuses cap out too soon or if even a doubled legacy bonus isn't all that great, then meh. +5 on the home continent might convince the Aztecs to bother someone else first, but otherwise seems meh.
Anything with a unique improvement? If these are not significantly better than farms or mines, or if they come too late in the game, there are better ways to get these benefits. Ziggurats, Sphinxes and Great Walls compete with other good improvements. Stepwells come late. Etc. All require food, housing and amenities to leverage. So these are not automatically awesome, or even good.
China? Using builders to rush early wonders sounds great! Except that such wonders come at the expense of other builds, such as defending against the Aztecs or barbarians, improvements, settlers and districts. Worse, there aren't all that many early wonders; this ability probably boils down to "China can get one extra wonder." Which is not bad! Hardly OP. OTOH, the Eureka/Inspiration bonus seems very good: If China misses half of these and gets half of these, that's a 7% bonus to civic and tech advancement.
Conversely, the Aztec builder ability for districts seems to have gotten lost amidst all the other Aztec goodies, the bonus lasts all game, and every Aztec city is going to have a few districts. Taking civics for +30% production on builders and +2 charges per builder better than halves district costs when using builders for this, though an Aztec player might have other plans for his civics. Between extra amenities, bonuses to combat strength, and using builders for districts, the Aztecs have a complete toolkit to last them the entire game.
No civ can be considered OP if it cannot weather an early onslaught by nearby civs with early game military advantages. A civ that folds in the face of three Eagle Warriors or War Carts is hardly a threat!
Brazil? Rainforests are great, but will probably apply to very few cities, perhaps only one.
Kongo? If their fake neighborhoods can only be built amidst the trees, many city locations become very difficult to use, because there are no trees.
England? Possibly the realization that best corresponds to history. Not much before the dawn of the Industrial era, but if they survive that long, they have the perfect toolkit to sweep the world... but this advantage will erode. Hard to say if this tradeoff is OP or maybe UP. Of course, if London starts near another continent on the same landmass, an early settler build or three also provide extra units. Hard to say no to that.
Egypt? Iteru seems solid but not awesome; when it comes to building wonders, Egypt is never better than second best until the modern era. Their chariots are not special for an early UU, though having an early UU is downright useful. Trade route bonuses? Hard to say just yet.
Germany? Probably very good. An extra military policy is great, a unique industrial district comes late but is also great, and an extra district also great. The bonus against city-states, and the u-boat seem lackluster, but who cares?
Scythians? They get bonuses versus wounded units, which means they almost always get bonuses, because it usually takes more than one attack to kill something, so their hordes of light cavalry, available throughout the game, produced at 25% cost (2 for 1, civic for +100% production) are likely to be even more effective than expected... which is saying something. With that kind of focused advantage, I expect them to be very scary in the hands of a competent player, or the preferred first target of an early rush. Which? Hard to say, because we don't yet understand pacing. But I suspect that it will be difficult to exist on the same landmass as a fully armed and operational Scythia.
France? Bonus for wonders during the entire midgame (or midgame and endgame, for games that never make it to modern times) seems very good: One civ4 player here sigged that 10 Axemen is the best early wonder, and it may be that sinking early production into a risky wonder instead of settlers, builders, units and maybe a district or two might not be so great. An extra, early spy? Depends on how good spies are. UU? Far less good than Redcoats, unless London is on your continent

.
India? Depends greatly on whether opponents spread religion to you, and on whether you can stay at peace. Their UU seems great for the front line, with ranged units in the rear also benefiting from the decrease to enemy strength.
Sumeria? Not so happy when the barbarian camps are gone. Not so happy once the War Cart becomes obsolete. Awesome if the advantage gained is sufficient. OP or UP? We won't know for a while.
Japan? Hard to say. Half-price on a few districts combined with extra adjacency bonuses from districts seems good, but the effect might be too subtle to matter. Coastal bonuses? I suspect a skilled human player will make these shine, on some maps. Overall, who knows?
Greece seems at worst solid and at best awesome. Wildcard policy? Gotta be great. Early unique district? Probably great. Bonuses from city-state suzerainty? Can't be too bad and maybe fantastic. Hoplites seem very good, but there are many early units: Some come earlier and some seem more powerful.
Rome might be OP but also might be UP. If legions are too good, Rome becomes a great target for early extinction. But of course the AI won't be smart enough for that! The extra monument seems really good: In a sense, Rome has a faster start than Sumeria. (So do the Aztecs, whose starting Warrior is an Eagle. Eagles > vultures?

) All roads lead to Rome? Might be great, might be meh.
Then there's Spain: Very scary once they have a religion. Unless they have to sacrifice too much to get one, or are wiped out before they can capitalize on that advantage.
Etc.
It may be that the entire conversation of OP vs UP is meaningless: Against the AI, each civilization provides a unique advantage that the AI doesn't know how to compensate for, allowing a human player to dance around drooling AI civilizations. Against humans, the only truly important advantages are those that allow survival past the early game.
Or maybe not! We'll see.
Anyway,
Ken