As far as warriors go, none have been more successful at conquering the world than the Mongols. They controlled more territory and have won more battles than any other faction in the history of civilization. They also defeated Russia in winter, a feat that could have changed history had it been repeated by the Germans in World War 2.
But they were also a truly organized war machine with incredible martial acumen, and not just the barbaric horsemen they're often portrayed as. They'd definitely give the Vikings a run for their money in the landscape of For Honor.