Okay, here are my ratings.
The Rat - weakest. A total wimp.
The Sultan - very weak. Likable scum. Not much of a threat.
The Pig - very weak. He will build up an army of spearmen, but obviously not too threatening.
The Abbott - weak. Builds massive number of monks, but any macemen, swordsman, or even a number of archers can take them out easily enough.
Nasir - medium weak. He can be a major pain if he doesn't defeat himelf. Poor economic skill often causes him to starve, but it he doesn't he can hit you with many dirty tactics.
Philip - medium weak. Arrogant Frenchman (of course). He builds knights, but has little else to protect himself and doesn't send an imposing army at you.
The Marshal - medium weak. Knights and swordsman, but not too many. Vulnerable castle defenses make you feel sorry for the once great knight; now senile.
The Snake - medium weak. Dirty tactics, scaling ladders, but not too great a castle and never too strong an army. Patience will take him out.
The Caliph - medium. Dirty and a pain in the behind, but not a major threat. He will set fire to anything he can, so invading his castle in the bigggest problem. Beware pitch and fire.
The Sheriff - medium. His castle is tougher than it looks. If you can avoid the fire, I build towers and connect them at the rear entrance, making it a short fight, but his fire will do major damage.
The Wolf - medium hard. In a good environment he is a solid opponent, but he can starve himself out of the game in a desert economy. If he doesn't he has a massive castle and good armies.
Wazir - medium hard. Good little castle and dirty tactics, but his armies are beatable with any good defenses and numbers (along with seige equipment) will take his castle down without too much trouble.
Frederick - medium hard. He is hard to figure out. He builds good armies and ammasses large funds of money, but he seems to fruggle. Whenever I take him down, he still have 10,000 gold in his pocket. Not much use if you don't spent the money!
Richard the Lionhearted - hard. The best of the Europeans, his castle is small but tough and he makes good armies.
Saladin - hard. Arguably the best of the desert rats, Saladin's castle is fairly strong and his armies equal to the task.
The Emir - hard. Sometimes he look better than Saladin. His castle can be taken with good seige equipment, however. He will attack often, and with swordsmen, but the armies are never too large.
Any one disagree? I would sure like to see a killer AI. Perhaps combine the Wolf's castle building with the Caliph or Sheriff's love for fire. Get Frederick's economic savvy, but spend the money on soldiers instead of hoarding it. Finally, add the hit and run tactics of the Wazir or Caliph with the seige tactics of Saladin or Richard. That would be a good opponent.