If based on conquest, the mongols have no competitor. And germany is not a country with no resource.It has abundant coalfield that powered its industrialisation. and the german tribes is all over europe. The angles and saxon in UK (angleland becomes england), The franks in france. The bumiputras( celts, gauls,gaelic) were overwhelmed. The vikings came later to whack the german tribes.
But the mongols, they were already at the Danube after crossing russiaand hungary and would have invaded europe if not for the death of their "khan".
Greatest is evaluated based on what that race/people group has done for mankind