Given that it took an army of heroes/villains from the DC universe with prep to take down Imperiex, and still a lot of them died, I would have to give it to Imperiex in the end by a slim margin. He is certainly up against some very formidable opponents so it is hard to call.
Darkseid cracks his armor with OB normally ....
Cracks Imperiex's armor:
With enough prep Darkseid can recreate the soulfire formula and take Imperiex himself.
If this is Nu52 Darkseid, then Thanos does all the work with the IG or Cosmic Cube. Despero is the odd man out
@Ancient_0f_Days: Those scans dind't showed the OB cracking Imperiex armor alone... It took combined effort to do it.
You see the blue energy, apart from OB, thats actually from Kismet, the DC version of Eternity. I think its safe to assume, given Kismet's power level vs Darkseid's, it was her blast that did the major damage.