God knows what's up with Captain Atom after Rebirth.
Placing his New 52 portrayal is tough. In his own book, they made him into Doctor Manhattan in terms of his versatility. And yet, despite seeing uber beyond all other heroes at times in his own book, his actual place relative to other DCU heroes was unclear.
From what I remember, Firestorm was basically treated as his clone or equal in power, with Cap having more experience.
Major Force was his rival, though I'm not sure how Major Force stacked up to Cap in the New 52.
Now Firestorm has never seemed like the toughest guy in the New 52 despite having enormous power in terms of scale and scope of abilities. And Superman pretty much dismissed Major Force in the New 52, IIRC.