I'm gonna have to say Russia and Portugal... Russia has the strongest heavy armored horse archer in the game, Berdiche Axemen are uber (Or will be, presuming CA fix their horribly slow animations), and even at the beginning they have very strong troops. Their strategic position is isolated by virtue of A) being a corner of the map and B) being surrounded by a huge buffer of rebel territories.

They have easy access to the amber fields, and Kiev is a high income province (You should be able to take this easily before Poland or Hungary can if you head straight for it at the beginning of the game, then move east after solidifying your western lines. If Turkey gets Sarkel before you get there it's no big deal, just means they'll have to deal with the Mongols instead of you ;) )

Portugal has the uber combination of musketeers and super elite pikemen... Plus Jinettes for awesome light cav and missile cav in one package... In terms of late game power Portugal has the "perfect" army. And of course they have a great strategic position, wipe up Spain and the Moors and you only need to station an army (Or a fleet) at the land bridge and you're all set on that side.

The moors are surprisingly good as well. Desert Cavalry, Granadine Jinetes, Tuareg Camel Spearmen... Easy control of the gold and ivory in Timbuktu and Arguin... Really good position, much better than the Emoheads in MTW.