I think it might be Zerg, as the collective would eventually adapt to flood infestation as the do to other things such as bullets and tanks. Not to forget that it’s possible for the Zerg to “out-infest” a world with the flood, meaning it would be close. Although, if a single flood infection form infects a zergling, it could corrupt an entire hive of Zerg and turn it against the collective of Zerg. So it would likely be the longest stalemate of all time, the ultimate tie.