The Star Wars universe faced an enemy that I think is more dangerous than the Flood. Have you read Fatal Alliance, during the old republic era? Some hex droids that were nearly unstoppable. So deadly that the Sith and Jedi and Republic had to team up to take it down. In Halo, during the modern era, even though the Flood were loose, the Covenant and Humans didn't really team up to face the threat, and they still pretty much prevailed. So I'm not sure, but I think those droids might be more dangerous. Not to mention the zombie/flood like infestation that was talked about in the book called Red Harvest. I think that's the name.