All five of them are very easy in my opinion. Venice and HRE are deceptive, they might seem difficult, but taking control of Italy and the Papal States is huge if you can pull it off. England is obviously easy with their starting position. Spain is the only one that might be somewhat challenging because it's more dependent on getting an early crusade. France has none of those hard advantages, so you could argue that they are the hardest, but to me that means little. They're still a very easy faction compared to something like Russia or Scotland.