Akkad [17]
Antananarivo [19]
Auckland [19]
Babylon [17]
Bologna [19]
Buenos Aires [14]
Brussels [16]
Caguana [16]
Cahokia [18]
Fez [2]
Geneva [15]
Granada [16]
Hattusa [1]
Hong Kong [7]
Hunza [13]
Kabul [16]
Kandy [1]
Kumasi [16]
La Venta [16]
Lahore [18]
Nan Madol [19]
Nazca [4] (7 - 3): Nazca Lines can be very good, but they're not particularly useful early on. You really have to wait until the Medieval/Renaissance Era to get any use out of them. On top of that, they're exclusive to desert, which as we all know isn't a particularly desirable terrain type. If you get Petra and make it to the midgame, yes, you can get some impressive yields. But even then, placing the Nazca Line effectively removes a workable tile in your empire. Now, is that a death sentence in a metagame where you don't grow especially large cities? No, but even in the best circumstances, the Nazca Line always has at least some asterisks next to its bonuses.
Ngazargamu [8]
Rapa Nui [19]
Taruga [11]
Valletta [18]
Yerevan [21]
Zanzibar [20] (19 +1): When I think about the best bonuses in Civ, the "S-Tier" bonuses, there's two metrics the bonus has to meet: is the bonus game-changingly strong, and is it consistent enough that you can rely on getting that bonus? Zanzibar hits both those marks with flying colors. First off, just becoming Zanzibar's Suzerain grants you 12 Amenities (6 from each resource they give you, Cinnamon and Cloves). That kind of Amenity injection is only directly comparable to the Colosseum, which has a really strong argument for being the best Wonder in the game. If that wasn't enough, becoming Zanzibar's suzerain also gives you two copies of these resources, allowing you to use them to barter with other Civs. This means that not only is Zanzibar giving you a ton of amenities, you are also getting a diplomatic bargaining tool. You can turn this into extra money, resources, or favor from other Civs, and you'll get a small relations boost from using the resource to trade. Oh, and you get a lot of gold from them just by their nature as a Trade City-State. Whether you're playing a peaceful or aggressive game, there is no situation where Zanzibar is not a huge boon to have under your wing. Really, they should probably take the whole competition.