If its a conventional war with each race at their height of power then the only two real contenders are the Tyranids and the Flood. The Xenomorphs don't exactly rank high up on the cosmic food chain of assimilation or hive-mind galactic threats. The Zerg certainly can be as big a threat as the Tyranids though so I don't think they should be discounted here either or taken lightly. You have to remember that the Flood are essentially the genetic aftermath and parasitic remnants of the Precursors eldritch corporeal remnants in the Halo universe after the Forerunners betrayed and tried to obliterate them for passing The Mantle onto ancient Humanity instead of them. They are essentially undead constantly evolving gestalts of collective memories, minds, personalities, and technology. The earliest and first Grave Mind was able to fully corrupt super advanced AIs designed to COMBAT them despite their synthetic nature making them immune to assimilation by the Flood.
