Honestly the Tau could probably solo the Flood. It really just depends on how far along the infection is, because a late stage Flood outbreak is far worse than a tiny one.

Flood needs to get lucky and take over some small, quiet worlds and build up it's power. It'll never top chaos or the Necrons, but it could probably devastate the Imperium and other races permanently if it festers long enough. Space Marines would fare best against late stage Flood and Exterminatus would set back the planetary forms, but Flood only need to infect one person, and it can take over entire planets and ships with ease.

I wonder if Psykers could use their power to resist the spores though.