Who wins?
Captain Atom should win this with his ability to quite literally make a Big Bang, also, he can probably take the Power Cosmic away like he did to the Flash and Firestorm.
It's not as simple as that. Silver Surfer has fate hax that could screw over Captain Atom's power nullification. Surfer can also just, do the same thing to Atom. Plus he also has hax that Captain Atom can't resist or even if he could, he can't resist it that well.