If you are talking what country is the most powerful to play as a human in Single Player mode, it depends. If you like early power, you can be either Spain or Portugal; Spain is slightly easier because it only has to take 5 additional Iberian provinces to hold the penninsula, whereas Portugal must take all but three. But Portugal gets to start its explorations WAY ahead of Spain, which is a real plus when you are sending explorers to places YOU want them to go (e.g., the Carribean), not where Portugal actually sent them. You can build up quite an empire quite quickly with either country, and can knock France out of the running early on, leaving only the BWB to deal with.
In late game, it has to be France, simply because of the leader file. Beating England early is easy, the slow but steady rise in power as you consolidate all of Western Europe isn't tough and lays groundwork for a huge ass country, and you will get to explore and colonize eventually. Then, when the leaders kick in, you just power through all opposition.
I'd have to put Austria up there pretty close to the top, too. You end up with tons of provinces doing nothing but waiting, then get to wipe out both east AND west of you. Of course, you don't get to do the colonizing thing so easily, meaning you have to take maps from neighboring countries like France and Spain, but who cares when you can just plow your way through Turkey and the Near East, as well as Russia and the steppes?
If you are asking which country does best as an AI nation, left unmolested, then unquestionably it is Austria. They always seem to gobble up most of Central Europe, and force you to deal with them early on to avoid long tough slogging later. If you are playing England or Portugal, it's really hard not to eventually face a BWB.
If you are talking MP, then I'd have to go with Sweden. Damn files are stacked in your favor, no surprise. :rofl: