I'm going to go with Flood. During the Flood's feral stage, the Necromorphs would give the Flood a run for their money, but once a Gravemind is established, you've essentially got a species that has the cumulative knowledge of everything it has consumed, and is committed to total war. Even if the Necromorphs consumed two of the three solar systems, the Flood would much more effectively utilize all biological mass. Having never finished (actually hardly began) Dead Space 3, I don't have all the facts, but you have to understand that the Flood destroyed an entire galaxy in 200 years, and the only way they were stopped was by a galactic super weapon.