Flood. They are parasites and really no one can kill them. Didn’t they force the forerunners to kill themselves?
Yes, they did force the forerunners to kill themselves.
The prometheans annoy me…
They did fight against the flood with more lethal weapons. I go with the Forerunners/Prometheans.
Considering that the Forerunners had problems with defeating the flood I would say the Flood wins, for now lol.
Prometheans for the didact 
Seriously for their story 
The flood winning bugs me since the Prometheans are pretty much robotic based life forms, they look more like machines than creatures. And I would assume that flood cannot use such a host. In terms of fighting them though, I’d say the flood is easier besides Halo CE flood on legendary especially on the Library.
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> Flood. They are parasites and really no one can kill them. Didn’t they force the forerunners to kill themselves?
Yeah, history says so.
Prometheans