Roster-wise, I'd probably say the French and the Turks... but in actuality, the French have to maintain 4 fronts most of the time (Milan, HRE, Spain, England/Danes), and of course the Turks have to deal with the Mongols and Timurids...
Spain is probably the most powerfull all-around faction. Easy borders to defend, very good units, no crippling weakness, except in the ranged department pre-gunpowder... but then again they have Jinetes so it's a toss-up.
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