Babylon: Ingenuity - 50
Carthage: Phoenician Heritage - 29
Greece: Hellenic League - 32
Korea: Scholars of the Jade Hall - 41
Poland: Solidarity - 49
Portugal: Mare Clausum - 24
Siam: Father Governs Children - 28
Sweden: Nobel Prize -11
The Inca: Great Andean Road - 43
The Maya: The Long Count - 36
The Shoshone: Great Expanse - 25
The Zulu: Iklwa - 27
Correcting for Amarr's Babylon vote. The vote is +2/-4. I'm a bit confused about your justification with Siam though. Could you explain it a bit more? It just means you need to spend gold on city states to get them to to friend or ally statuses. The benefits are pretty huge when you can do this. Assuming you were comparing it to the Greek UA (Greece doesn't have a UI), I think they are completely different. Greece gets you city states easier, and allows you to keep them with less effort. Siam gives you more benefits from them once you get them though. I think Greece is probably better early game, while Siam's ability pulls ahead late game.
Sure, I'll elaborate, but expect this to be a very lengthy one.
I picked Siam's UA to be the worst because it's almost completely eclipsed/outclassed by Greek UA. And here is why.
There are many things a CS ally/friend does for you. Siam only focuses on 3 and ignores the rest.
Let's look at what a CS ally/friend does for you and how Siam compares to Greece.
1: Gives you non-strategic resources. Food, Culture, Faith, Happiness (+3), Science (after adopting the appropriate tenant)
2: Gives you luxury resources
3: Gives you strategic resources
4: Votes for you in world congress (You control their votes)
5: Fights for you in a war.
(5.5): Locks whoever you're at war with out of their benefits from CS friendship of your allies. Deny them opportunity to ally the said CS.
6: Gives you Great People (after adopting the appropriate tenant)
7: Allows you free passage through their lands
8: Gives you units (militaristic only, unique units too!)
9: Adopts your religion twice as fast (After selecting the appropriate belief)
And there are probably many more indirect ones. But for now let's just focus on these main ones, and look at how Siam does vs Greece.
Purely mathematically speaking, Siam's UA covers half of #1 and none of the rest, functioning at half efficiency compared to half of the Greek UA.
Ok it's not as bad as it sounds in the last sentence. That was just a lower-limit of how Siam UA compares to Greek UA. And the lower limit is 1/2 * 1/2 *1/2 *1/9 = about 1.5% of Greek UA's power. I would say in reality it's maybe 30-40% of Greek UA's power, but once again, that was a lower limit.
Let's keep looking in detail.
First, Greek UA has 2 parts. Influence drop rate halves, and Influence doubles rate doubles. Siam has no answer at all to the "influence recover rate doubles" half of the Greek UA. How does this make Greek UA superior? Well, pledge of protection activates to +5 influence twice the speed. Patronage tenant activates to +20 influence twice the speed. If you happen to be completing a quest for a CS during that time, that directly translates to Excess (the leftovers after the 60 ally mark) influence.
Now, I agree with your generalization of Siam UA vs Greek UA. Greek UA allows you to get CS friendship/ally easier and maintain it easier. While Siam UA does not make it easier than any other civ but does grant you more benefits once you befriend/ally CS.
Because Greek UA halves influence drop rate, mathematically speaking it doubles all yields per influence (+100% for those of you bad at math). Siam only gives +50%.
I will now show a practical example, demonstrating this.
Here's a perfect scenario for Siam. We're looking at 1 CS. It has no luxury resource nor strategic resource. It is maritime and you have 4 cities, meaning it would give you 3+3*1=6 food per turn while ally, and 2 food while friend. There's nobody else to war with. It's way too early for world congress. You have no tennants or beliefs that affect the CS. Basically in this scenario, nothing matters other than how much food it's giving you.
I'll show you that even in this perfect scenario for Siam it's still outclassed by Greece.
You start at 5 influence from pledge of protection. You pay 1000 gold to gain 115 influence and end up at 120. Your influence drops at 1.5 per turn. You can not put any more money in later since you need your gold for other stuff. All these values are normal for a fast speed game. But it really doesn't matter.
As Siam: Your influence drops to 60 in 40 turns, and to 30 in another 20 turns. You get 40*6+20*2= 280 food total. Your UA bonus makes that 280+280*50% = 420 food. You paid 1000/420= 2.38 gold per food.
As Greece: Your influence drops to 60 in 80 turns, and to 30 in another 40 turns. You get 80*6+40*2= 560 food total. You paid 1000/560= 1.78 gold per food.
As we can see, Greek UA of having influence decay is mathematically equivalent to doubling yield.
So even the part Siam is good at, food, culture, and faith, are beaten by Greece.
And now, look at the list up there. Look at how many more things Greece is able to get for twice the duration. Siam UA does not affect happiness for example, nor does it affect luxury/strategic resources.
Greece will be able to sustain 2 CS for the price of 1. Siam will be able to sustain 1 CS for the price of 1 but get 50% more yield only out of 3 resources.
The reason I said Siam UA is self contradictory is that, sustainability won't be an issue only if you can permanently keep the CS your ally. Which would either mean lots of money or Gunboat diplomacy. Both of which are late game (if it even happens to begin with). However, in the late game food/culture/faith are much less significant compared to other benefits of allying a CS.
There you go, a detailed explanation.
Combine with freedom or autocracy for a more potent effect. 
