With prep I'd say the Reach. The Shi-ar could match them in terms of ships and technological baseline imo. However the real battles would be fought on the ground where single Scarabs (with hosts of course) would pose threats to most shi-ar imperial guardsmen sans Glads.
I'd actually give the Kree a better chance against the Reach.
The Reach have cool tech, overall above that of the Shi'ar, however they have far less worlds, iirc around 80, to the Shi'ar's 1,000.
I'd say the most likely outcome is the Reach lack the resources to expand much into Shi'ar territory, but have enough military power to make it too painful to take over.
Though if it was a real fight to the finish, Shi'ar win, despite a massive body count.
DCnU? I have no idea what they're like there, I can only speak of classic.
Pre-FP, they've got less power than the GL corps, but their weapons (or at least, those on the Infiltrators) can find weaknesses and they've got a lot of tricky tech, so they can hold their own against stronger foes pretty well.
Shouldn't be much of a fight on paper. Even ignoring the vast difference in the size and scope of the forces, the Shi'ar could just use the Starcracker on the star of whatever system the Reach happen to be in... and then go about their day.
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Doubt they'd have the time unless the Shi'ar sent them a tweet warning them about what they were planning on doing. By the time the star went super nova it would be too late and the force of the explosion would tear everything to shreds.
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I'm sure they have tech that can pick that type of stuff up. I think most advanced alien tech do. Dominitors for one had that. And if they blow the star sytem they are in, Shiar would die anyway. They're not into suicide last i checked.
The Shi'ar have forethought on their side, they can stargate out of the system the moment the activate they implement the Starcracker. The Reach need to wait until they get the data, and then they need to react... and I fell like by then it will be too late do anything. Maybe some of them get away, but not enough to matter.
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