My first choice would have to be the Black Knights. The heavy manpower, heavy cavalry, and large fortresses, not to mention a good navy, make them unbeatable. With 6085 and 6086 in the equation, it would be virtually impossible for them to be defeated in both, unless everyone gangs up on them. (My only worry.)
Second would have to be the Crusaders, due to the manpower that they possess. What puts them over the top is the civilian edge they have- lots of quality weapons coming from the armourer, and they can hang out at the Guarded Inn after a victory.
Black Falcons next, since they have good manpower, and solid fortresses. Not over inspiring, but they can get the job done.
Fourth place would go to the Dark Forest/Forestmen combination. The number of archers are a plus, as is the forest complex they hide out in. The lack of real fortress would be their undoing, as would their big weakness to the Dragon Masters. (Can someone say firebreathing!
Finally, I would have to pick the Dragon Masters, since they have dragons capable of wreaking havoc and destruction, and a wizard capable of wreaking havoc and destruction. However, their shoddily constructed hideouts could be decimated by a big army like that of the Black Knights, while it only takes a couple of arrows to take out the dragons. (Like Bard in The Hobbit.)
There's my top 5: Let the Battles Begin!