What exactly makes Doctor Doom such a credible threat? Not criticizing, but actually asking because I'm ignorant of him since all the F4 movies we've gotten have done shit with making him look super badass
I don't know much about either one, but what little I know is that Doctor Doom is much, much smarter than Thanos. Unlike most characters, he also doesn't restrict himself to one specialty. He's a genius who creates amazing technology, but he's also an extremely powerful sorcerer. And I'm not positive about this, but I think the army of Doombots he has are also capable of that same sorcery.
He's also just really strong mentally.
Also, read Secret Wars to see how big of a threat he can be.