I think I jumped in too late to give a vote, but what the heck I'll throw in my 2 cents anyways. The best starting factions in order.
1. The Spanish. They have good units, good income, really good income when you figure out trade, nice starting location, and a built in enemy to whomp. You can go the Almohad, Egyptian, Turkish route for years and never ever hear a peep from the Pope.
2. The Byzantines. Incredible money (think of being the medieval version of Bill Gates), great units and no Pope to worry about. These guys would have gotten the top spot if it wasn't for the units half-life. The units become outdated after awhile. I would think that even a beginner would end the game way before this becomes a problem, but it has the potential to become a problem. The Spanish don't have this problem, they continue to get great units all the way through.
3. I know this is going to throw a couple of people. The Italians. These guys are another great money-maker, they have some great units (Light Italian Infantry), but their position is only so so. However, if you become a great naval power that won't really matter. These guys might take a little planning but I believe they are the next easiest.
After that, I would say the English (they are my fav Catho faction), the Danes (who get even better if you have VI), and any of the Muslim ones. I seemed to have a natural aptitude for the Turks, but you may not.
A couple of stay away froms. The afore mentioned HRE. These guys are the toughest faction to play, evryone wants to have a piece of the HRE puzzle. Stay away from the Russians in High. With the Mongols breathing down your neck it takes alot of planning to survive the first 30-40 years.
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