Songhai and India are good right now, but Siam is so far out in front post-patch that I am having a very hard time thinking of reasons justifying the use of another civ. The old Babylon gameplan remains 100% applicable for Siam. It won't resolve in 200 turns, but it will work fast enough to win on Deity.
I just (finally!) did a 181 turn Deity spaceship with Siam. This is my first time to do it on Deity, but it is tainted by my abusing absolutely every exploit. I have to admit, I was getting desperate to get this in before the patch.
The start went extremely well: 2x grass+cow start, AH on turn 1 from a ruin, 2x culture ruins.
First thirty turns were REX while teching to HBR
Capital: Scout/Settler/Settler/Settler/Settler (size 2)
City #2: Partial Warrior/Settler/Settler/Finish Warrior (size 2)
City #3: Worker (size 1, nearly size 2)
City #4: Warrior/Partial Warrior (size 1)
City #5: Warrior (size 1)
City #6: Partial Warrior (size 1)
By turn 32, I had six cities, 1 settler, 1 worker, 4 warriors, and 2 partial warriors. I was at -9 Happiness (found 2 Natural Wonders). I bribed Sully to DOW India for ~230, and then sold City #2 to him for ~890. I bribed Rome to DOW Sully for ~750 and sold City #4 to him for ~1100. Gandhi wouldn't DOW anyone, so I only got ~500 for City #5.
On the next turn, HBR finished, so I used the 4 warriors to retake City #2 from Sully, settled City #7, and bought 4 horsemen, which left me with about ~200 in cash. Turns 35-50 were spent conquering the nearby AIs, leaving them with only the cities I had sold them, all touching in a triangle. They all sued for peace leaving me everything.
I teched to Education while building enough settlers to push me back to -9 happy. Three RAs quickly signed. I started directly on the Wat in my core cities and buying the Library when I could afford it. The fact that Siam can build these out of order is a big deal.
I met Hiawatha and Nappy and bribed Nappy for a DOW. Neither ever had enough gold at this point to bother selling a city, so they got my excess luxes instead. I screwed up the first round RAs, since I was concerned about finishing Compass in time. Archery finished in one turn, so I got Mathematics, Acoustics, and Astronomy and picked up Freedom. I re-signed the agreements after subsidizing Sully. India and Rome could barely afford it.
I did some minor expansion, allied some Cultural CSs, met Wu, backfilled techs, and researched Navigation. When the second round of RAs came in, I got Archaeology, Scientific Theory, and Chivalry, and I bulbed Biology and Banking. I popped a GG for a GA, picked up Communism, started the FP. I sold Wu one of my core cities for ~2500 and allied two Maritimes and bought Settlers. I retook the city, moved the horses in Wu's direction and then upgraded to Elephants. They began chewing up the stream of troops coming from Wu and working on a second GG. I started settling tons of cities with a Colosseum/Wat/Library build order.
Raw research and expansion until the third round of RAs came in. I dodged a 25% chance of missing Radio and was able to bulb Flight, Radar, and Rocketry. Apollo started on turn 147 and finished on 171. A GG, Chichen & Taj gave me enough GA to power through the end. Nappy and Hiawatha also came after me, but it was too late.
Cheesy, but done.
You are right about the power of Siam. They have several things going for them:
1) Wat not requiring a Library. This gives you a bunch of flexibility, and allows you to build the Wat directly and buy the library later when you have the population to staff it.
2) Culture...extra CS culture and the +3 from the Wat really add up.
3) The Elephant is good enough to stand up to riflemen. They can fight a defensive war against the AI when Knights would be too weak, and they last long enough to hold out until Cavalry are available. This lets you play an entire game without distracting yourself with foot soldier techs.