india is geared toward a smaller peaceful empire
A less obvious combination I'd like to try is Persia and China. Start Golden Ages with Great Generals FTW.
Denmark's increased embarked speed + England's increased naval speed
Deserves a prize for most creative, even if not the strongest.
I got another fun one: America and England. Longbows in the frontline can now use their full range. Think about it for a moment.
Seriously though, I would stick with Siam and add Babylon UA. You get about 3 more great scientists throughout the game; I wonder when I can launch the spaceship?
Some that haven't been mentioned:
England + Polynesia: Reveal the whole world before 2000BC!
Spain + Polynesia: Chances are, you'll be finding the Great Reef and Krakatoa unless someone conveniently started right next to them.
France + Russia: A bit of a stretch, but watch as your city borders engulf your opponents once you get Kreposts running. Also, cities. EVERYWHERE. And with a nuke in each for good measure.
rome is geared toward a larger empire warlike empire while india is geared toward a smaller peaceful empire
Actually, if you're burning great people for golden ages, China and Persia would work better. Plus you can warmonger during a Golden Age with a Great General and then burn it for another golden age.
Imho India is best for big empires with big cities. At the latest in medieval when cities are growing, you getting enough production to build enough buildings, and can further growth in size without having that problems with happiness. So the bonus together from rome would make a very strong nation, based on the infrastructure.