Preboot
Atuma but if OM is fast enough with his spells he can take him.
Ocean Master, DC Atlantis preboot consisted of several cities with superpowered races that brought different stuff to the table from magic to tech to numbers etc.
I don't know, OM had some big influence even altering reality at some point and being in the inner circle of villain groups with Luthor, Libra etc but Atuma has also threatened Atlantis leading armies etc.
Postboot
I'd take OM in all three.