I haven't read enough comics featuring solo adventures of Hank and Nate Heywood to know their arch nemesis.
I'm pretty sure Hank (Commander Steel) had at least one because he had a solo series in the 70s
I'm pretty sure that if you read this series there will be one or more supervillains that were his arch-nemesis.
^looking through the appearances, he just has a few one off enemies in that. The Steels have fought a few villains like Ivo, Magog, and so on, but usually as a team and not individually. Baron Blitzkreig apparently ran into the first one, but they still didn't fight a ton.
Nazis in general.
What do you think?