Tough one. I'm going to say Covenant. True, the Empire has the Death Star, but those things are expensive. The Covenant can more easily leverage some heavy firepower.

I doubt the Covenant would have a REAL problem taking down a Death Star, if they have a big enough armada. They'd take heavy casualties, but eventually the Death Star would become a floating mass of slag.

A final key element is that the Empire would probably underestimate the Covenant long enough for the Covies to get a star map, and glass Coruscant.

And a Jedi/Sith will not win against the Covenant. Can anyone say "Hunters with Fuel Rod Guns"?

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What the photon?
-Ford Prefect, Hitchhiker's Guide to the Galaxy