Russia.
Cheap infantry, and once you smash the Khanates and Sibir, you have many, many options, from colonizing Siberia to having CB's on practically all of your neighbors for whenever you feel like a war. Also, being Orthodox, it's easy to align yourself with nearly any Christian alliance and gobble up chunks of Turkey when they DoW them

In every game I've played as Russia, I've ended up driving the Turk completely from the Balkans and making it mine (well, except for whatever my allies take.)
On the other side, playing Turkey and Persia is fun as well.
I hate playing France and Spain, it's way too easy from the beginning.