So far I've played Byzantium, Scots, Russia and now I'm in the middle of my moors campaign. I'd have to say Russia.

Great heavy cavalry (bit weaker than its western counterpart at charge, but not so much), great missile cavalry, great archers and melee infantry, and of course, the famous cossack musketeers.

They don't have a single weak unit.

Also, their starting position is great. A lot of rebel settlements. Conquer first the western ones to cut of poland and hungary and all your problems are solved.

Two problems that russia may have are mongols, and getting to america (unless you really expand westward). I've reached america from the black sea, and, believe me, it is a pain in the rear.