I'd have to say England, simply because of the starting position. Once you secure the British Isles, you can have (if you want) seven perfectly safe cities that will never be touched. Regardless of unit rosters and the like, that's such a huge advantage, especially as if you gobble up enough rebel provinces in France and Burgundy quick enough, you have a mainland empire the same as any other starting faction, in addition to your island kingdom.
Spain/Portugal are up there as well, for similar reasons. Not as good as England, though.
The AI always seems to do well with Denmark in the early to mid game, and then in my experience they'll be supplanted by a central/eastern european power alongside the spread of whichever Italian state has done the best (normally Milan).
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