I would give this one to Apocalypse. Apocalypses Regenerationn is going to be a problem for Atrocitus in my opinion. I also think that Apocalypse is slightly smarter than Atrocitus, so he may be able to find a way to defeat him. The final factor that makes me believe Apoc would win is his technology manipulation. Im not sure if Apocalypse would even be able to fidge with the Red Lanter ring, but even if he could mess with it a little, it gives him an edge, though i will admit this is only an adding factor to my vote, not the defining one.
So yea, i would say Apocalypse, but if anyone would like to proove me wrong, please do. I dont remember everything about Atrocitus, so i could have messed up, but im voting Atrocitus.