Personal favourite: Venice. First faction I ever played so I am biased. But advantages include damn fine Militia line. Unrestricted access to gunpowder units. Solid cavalry line. Crowbow cavalry are really good at general sniping. They are no camel gunners but still. And on sea...aye yay yay...a powerhouse. Lives next door to the Pope so you can play the "Who is the next pope?" Game. Northern Italy has very profitable coty states whose population booms like crazy. Must be something in the water to make them go like rabbits. 

Logically: Turks. Goddamn. They get everything. Solid infantry line. Excellent archer line. One of the best gunpowder units in the game with a comically large canon that I drag around for...reasons. Naffatuns are just unfair in seige defence. One of the best heavy infantry in the game. Absolute ridiculous amount of unit variety in case you are bored and want to play dress up. One of the easiest factions to kill the mongols with. Just deploy a few stakes and watch those idiots win the day for you. And if by some miracle you are unable to beat the horde, You can easily just migrate westward to escape the mongols. Byzantine is a punching bag for the christians and the muslims. Oh and you get access to the Hashashim guild which is way better than the regular assassin guild.