what are the most powerful countries in the game?
Well, that depends on the scenario. There is a load of different scenarios and one grand campaign (aptly called the Grand Campaign
In the Grand Campaign, you can play Spain, France, England, Portugal, Turkey, Russia or Poland. If I had my wish, Sweden and Austria would also be included (I am still fuming over that).
However, you can literally play any nation in the world by hacking the scenario file (which I hear is very simple to do). Greven's Papal game was such an edited Grand Campaign.
/Doomie