It depends what you plan on doing, i mainly turtle, so HRE is not ideal, as is Milan or France, Scotland is easy depending on what you do, if you can successfully boot england out of england, then it can be quite easy, but if you leave them in england, they become a pest.
I would say France is mixed, if you take over bordeaux, rennes and caen, and secure some clean frontlines, it could be entertaining, but i have never doen this successfully, so my opinion remains the same
Well, the easiest in vanilla are England, Scotland, Venice, and Milan. England/Scotland have the British Isles for a safe staging area, while Milan and Venice sit in Italy with enough available cash to make God feel Poor.
As for the hardest, I'm gonna have to say Russia, the Turks, and the Moors.
The Russians just don't have any money unless they beat the Danes to Stockholm and even after that, they end up having long distances between settlements (no good interior lines or army staging areas), better equipped enemies like the Poles make expansion into Central Europe difficult, and oh yeah, the Mongols arriving up your butt does nothing to help the situation.
The Turks have a lot of the same problems with little money, and some settlements that are so far removed from their main areas that they are almost a different country. If the AI knew how to use HA, they'd also have a problem with the Byzantines. As it is, they're out gunned in every department besides HAs at the beginning of the game, and right when that starts to change, the Mongols pop in.
As for the Moors, they've got that everyone hates them problem, and although they only have two possible fronts, Sicily taking Tunis almost always means they'll be fighting on two fronts. They're simply a light calvary dominated faction that only has javelin calvary as an answer to their better armored 'friends'.
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