Companion's are brutal.
Stronger, Faster, more GGs which means early wars will grant greater game-spanning military benefits. Or earlier Golden ages attained through conquests.
Greece are an ancient era powerciv.
Powerful companion cavalry, better defence with hoplites. Basically with greece, you want to do a bit of warmongering right at the start of the game.
China are going to have a very powerful economic edge with the papermaker, and England seem very balanced and defensive - extra range on longbows makes land battle and defense much easier, and they have powerful navies so naval superiority is a must for them.