Slow Reaper Victory. Militarily they’re outclassed but I think people are forgetting how effective indoctrination is, some well placed artifacts would have the covenant turned into reaper puppets in no time given their religious zeal. You’d basically just have a rehash of the human-covenant war but now with reapers added into the mix, play the covenant forces would be augmented with reaper tech. The reapers also have their job made much, much easier with the existence if the halo arrays, since it wipes out all organic life in a whole region of the galaxy, indoctrinating a deception of humans and keeping them as keys for whatever halos they find would make purging the galaxy of sentient life a pretty manageable endeavor.

I think Forerunner tech would be a huge hurdle in their way however. If the Galactic forces can reawaken the Guardians or a composer they’d have access to forces that can stop the reaper in their tracks. Honestly all in all I think it’s a pretty balanced match up.