Middle ages: Mongols, probably.
French cavalry was really disorganized and not effective.
English "Longbowers": Somehow overestimated.
Spanish conquistadors. Sorry, they are not middle ages. At that time there wasn't even a Spain around.
I would put my bet on the mongols at that time.
Although, once the issue is Middle ages, one might consider the Crusades. After all, they managed to conquer Jerusalem. Although it was lost later on.