I played the OCC quite a bit and I can win pretty much very time on Emperor. On Immortal its really inconsistent and I have to have the right start location (tile wise) and the right map location (neighbor wise).
Most time, the AI will leave you alone. I normally only keep 1-2 units around, (if I get bored I'll form up a posse and start killing stuff, but this will cost my turns in terms of time till victory. I think its the tech lead that you have that keeps them at bay and not to mention that on Emperor and below you don't need more than 2 units to hold back an AI.
Alexander is bar far the best that I've tried. I've done the OCC post patch in a fairly poor spot in 250 turns. It was along the ocean with mostly grass land tiles (so very little money). Babylon is my second choice simply because of the number of GS you can create. The quicker you get to Acoustics... Radio the better. Finally Egypt, most of your production is done on wonders so...
I have not tried Siam but I expect similar results to Alexander. Alexander though gets double the value for the "points" he earns with the CS while each CS Siam is friends/allied with is essentially 1.5 CS. So, Siam needs two CS for every 3 CS Alexander has, but Alexander will have say 1500G to ally with 3 CS, and Siam will only pay 1000G to ally with 2 CS, but Alexander will only have to pay another XG to upkeep in half the time. So, overall, I think Alexander will do best.
The most important thing in a cultural victory now, is city size. The large the better even if it mean sacrificing certain things like production and science early.
Edit: I actually tried Siam, definitely as good as Alex if not better. The unique building with two science slots is a big factor as well the fact that you may not find all the Maritime City states you want and thus you'll have an advantage here. On the other side Alex can rally more CS to his side and take advantage on the science bonus in the patronage tree.