I would vote for France, the armour upgraded Voulgiers wipe the floor with every Infantry (yes, even JHI) but for the danish Halberdiers, if you use the spearwall ability without hold positions. They are all-round units and cheap(!), good in offense (if you have the patience for a good ordered but slow attack) and defence. They also have a good starting position and can get rid of England very early in the game. The scottish guard and horse archers add a lot to their ranged abilities and they have some of the best armoured cavalry and pikemen in the game.
Milan is the only disturbing factor to them, but they are normally in war with every of their neighbours.

After all I think every faction is well balanced.