Siam is my favourite civ as well. It's a nice coincidence that a strong civ that suits my playstyle is also the representative for the part of the world whose history I'm most interested in.
I like playing with city-states, and also the flexibility to go for every victory condition with roughly equal bonuses to each. I also like having to work to some degree to use the UA to best effect, rather than having a passive bonus. All this is of course coupled with a very strong UU and a nice bonus from the UB.
Their UA & UB aren't anything to write home about, while the Greeks (who have a comparable UA have a very good UU and the broken-at-the-moment Comp Cavalry.
Considering the UAs comparable misunderstands and drastically underestimates the Siamese UA. The Greek UA doesn't really do a great deal - it saves you cash by leading to a slower rate of influence decay, but all those CSes you're holding onto aren't doing anything for you that other civs' CS friends/allies aren't. Siam saves you money as an incidental effect (by effectively giving you much of the benefit of allied CSes with friendship status, or the effective benefits of two friends with only one), but with cultural and particularly with maritime states is extremely powerful if you maintain allies.
The Hoplite is nothing special, and the Greeks lose impetus quickly since their units are both ancient era. Companion Cavalry are indeed very strong, but lose effectiveness once Machinery is researched and their ability to outflank and hit enemy ranged units is denied by enemies fielding crossbowmen who are their equals in melee. Siamese elephants are strong enough to win one-on-one against Pikemen with equal health, are better in melee than any equivalent-era unit (including French Musketeers) and remain solid into the age of riflemen.
If however you are going for a cultural victory or want to micromanage a small number of powerhouse cities then in my opinion Songhai is hands down the best choice. Make sure you have barbs on (maybe even raging barbs like I do) and adopt the first Honor policy asap: every cleared camp nets you 225 gold instead of 75. You have no idea how much this fuels development.
Playing on Emperor, as noted it's 75 vs. 25. This is however still a very strong bonus, and yes I've played Songhai as an economic powerhouse on Emperor and take Honor with them - it is an extremely powerful civ, and among my favourites (definitely my favourite for domination); however it doesn't suit my playstyle quite as well as Siam.
Paper Makers are excellent for China's economy. This, in addition to their GG bonus, make them good at two of the most important facets of civ (warfare and the economy). The economic bonus also indirectly helps other aspects of the game (culture, science, warfare).
A few other civs are better than China at individual things (warmongering, science, culture, etc.) but none have the whole package that they do.
Disagree. China is strong, but you're relying for their strength in theatres other than domination on the relatively minor gold bonus from paper maker, which doesn't bring them close to competing with economy-focused civs like Arabia, Songhai or Persia. I'd probably vote Songhai for 'most versatile civ' since they specialise in two things - gold and culture - that underpin all victory conditions, and both are complementary since the mosque is maintenance-free. Quick culture gets you through the Honor tree more rapidly and to the valuable gold-from-kills finisher, which boosts the economy further, money that can be used to invest in science buildings, culture buildings, units/courthouses, CS allies or whatever else you need, while culture can be ploughed into policies that benefit all victory conditions.
England is quite a lot better on a continents map, but I also don't see why anyone would vote for them.
Longbowmen. 3 range in the Renaissance is very powerful. It's misleading to judge a civ by its UA in isolation - you don't see many people claiming the Mongols suck because their 'bonus against CSes' UA isn't very useful. Same with, say, the Ottomans who have a poor UA but also the very strong Janissary.
Though I can't see voting for England, simply because a civ whose big sell is a unit with a good boost, and one that isn't game-altering (unlike, say, the Keshik) at that, isn't as good as one that directly helps you secure one or more win conditions. 3 range is good, but the longbowman is not Renaissance artillery - it has no particular bonus against cities and its attack is relatively weak, in an era when trebuchets outperform them as city-killers, and in an era with few units with movement greater than 2, being able to shoot from a long distance is less useful than it becomes in the age of mechanised infantry.