Favorite is Spain, then Russia. Don't know if they are the strongest but I like how they play. As Russia I sell excess Iron/Horses for a lot of early money to my buddies while still maintaining a strong army. The extra production is a nice perk too. I don't really care for the Cossack, in that I don't think it's that good of a UU (in IV they were awesome, because Cavalry kicked all kinds of butt and Cossacks owned other Cavalry as well). The UB is highly underrated, that thing really can expand your borders and, because it replaces Barracks, all military units are that much stronger.
Seriously, research Animal Husbandry and Iron Working as soon as possible, and then settle sites near Horses and Iron and build Kreposts. You'll soon have a huge army, all with one promotion to start, and rapidly expanding borders. You can become a beast.