Obviously the Reaper.
I mean they are unstopable without a super weapon and only one laser beam from them destroys a ship.
Also they can brainwash people and learn more and effectiv tactics after each time they wipe entire species from the face of universe. They collect example of us and replace things in their weaponary to operate more effectiv and aggressiv
Really impressiv
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But they were defeated by races about 50 years more advanced than we currently are.
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Jup because of their own technology. The the citadel and the portals are all their technology. They use it to navigate through space during their genocide against the current living species. We just ended a plan what strange aliens made up to kill them many cyclen before.