The best factions are ones that start out with sea provinces with trade goods or are able to conquer some in the early stages of the campaign. The most crucial element of the game is money and establishing an early sea trade network ensures ultimate success. Once you learn to do this, the game is quite easy, whatever the difficulty setting.
In fact the Danes are quite easy to play despite their meager start. They can take rebel held provinces around the Baltic,which means no wars with major power and all of which have trade goods and quicly become the wealthiest nation. And the AI likes to build ports even without trade goods, so a lot of trade becomes available without the Med.
I ended up modding my startpos file to put at least one resource and trade good in every province. That way the AI can trade if it builds ships and spreads them out a little, which it seldom does.
Kraelin's(sp) newly posted mod also changed a lot of the factors for AI building and deployment of trade so if you don't want to mod I recommend you download it. I haven't played it myself, I'm playing a campaign now and don't have a lot of time so it takes me some time to complete one, but I intend to try it next. (Or least read his files and steal what I like.)