Hmmm, looks like a few of my posts were deleted, including the one in this thread. Something to do with the crash?
Oh well, lemme readd my 2 cents. The question is "what was the most impressive ARMY during the middle ages", and though I can certainly admire the Samurai as a warrior, the armies of Japan never did ANYTHING very amazing. In fact, for the most part, they are "The Empire of Japan" only in name until they adopted modern technology.
The armies that stunned the world in the Middle Ages: The English for onesided victories, the Normans were certainly dominant for a time, the Ottoman Turks ought not to be overlooked, the Arabic armies shortly following Mohammed were certainly impressive.
Then we have the Mongols. Nobody came close to shocking and terrifying all the world around them quite like the Mongols. In NUMEROUS cases, they went in heavily outnumbered and won with barely any losses on their side. They had the most effective bow ... probably ever. They could move their whole armies at about 8 times the speed of any other army of the era. The two greatest cities at 1200 AD were probably Bagdad and Peking. They took and pretty much destroyed both. The also destroyed Kiev, Buda and Pest, and numerous other noteworthy cities. Nobody ever really stopped them barring the jungle terrain of Indochina and the fortuitous Typhoons that saved Japan. Their everyday tactics were brilliant and their proficiency in battle was astounding.
So yeah, my vote goes to the men in funny looking hats with goofy looking horses.