What civilization from, Mongols, Vikings, Africa, and Islam is your least favorite civilization, and why?
I'll go with the Vikings. I think that they were the ones who did the most damage and the least good.
Other civilizations, like the Mongols and Islam (in the early "Rise of Islam" period) at least created peaceful empires and/or great cultures. The Vikings did neither of these. There was never a "Pax Viking." There was no great period of Viking art or literature or learning. The Vikings were mostly a destructive force who did not bring any real benefits to the lands they attacked.