I've been thinking about this a lot.
Assuming some doofus actually flips the switch on the rings, that would mean that all organic life would be extinguished.
Aside from anybody on a shield world.
That would mean the only folks left would be:
Sky-Net's terminators, which are confined to only 1 planet and do not possess any space flight capabilities, Sky-Net wouldn't see any need to develop space travel as it only cares about Earth.
The Necrons, they're already dead. But they do have star-ships, and some pretty big nasty ones at that (look into "Battlefleet Gothic", one of the specialty games set in the Warhammer 40K universe). They can regenerate themselves, they're autonomous, they've got BIG ships, they can make reinforcements, there's a LOT of them to begin with, etc.
A few of the Eldar, specifically the Wraith-Lords and the Wraith-Guard. But they can't pilot ships nor can they create reinforcements. Plus, there aren't that many of them to begin with. So, once they're all gone, they're gone.
Then, there's the droid armies from Star Wars. They're not autonomous, they need a living controller to give them instructions, like a remote control toy. So, nobody to give them instructions means they'll just stand there like a bunch of high tech garden gnomes until they erode/corrode/rust, or whatever, into dust.
Now, how many troops and ships could you stash on a shield world, and how many shield worlds are there?
So, assuming the switch has been flipped, my bet is on the Necrons.
Sky-Net would never have chance, the Necrons could either just ignore Earth or turn it into dust from beyond the range of anything Sky-Net could shoot at it.
The Eldar wouldn't last very long either, Wraith-Lords and Wraith-Guard have got neither the range nor the firepower to even return fire on a Necron ship, even if that ship is in low-orbit.
As to the droid armies, the Necrons might use them as gigantic lawn-bowling sets, or just for target practice.
Now, how many troops and ships were on those shield worlds?
I highly doubt there were enough to take on the Necrons, and have a chance of winning, that is.
Even if the remnants of the Covenant and the human race joined forces, well, maybe they'd give the Necrons a good run for their money.
But I doubt they'd come out on top.
Unless, there were a LOT of shield worlds and they were very BIG and stuffed to the gills with troops and ships, but that would require several years of planning and prepositionning of personnel and materiel.
Meaning there wouldn't be enough "butts'n'bombs" to actually fight the war that's already going on between the Covenant and the humans.
I'm still workiing on my post about who'd win if the switch doesn't get flipped.
Later.