Definitely not Superman, he would tell that to literally anyone, even just a kid who got an A on his spelling test. Superman will say that about anyone that isn’t a cackling psychopath. It has no meaning coming from him.
Cap is tempting… that, and unlike Supes, he fought Hitler.
I pick Doom. If you impress Doom, you’ve probably redefined the limits of what is possible. Supes and Cap would be proud of you if you helped a little old lady across the street. Luthor would be proud for something just minorly evil. If you impressed Doom, you’re a fucking giant. ”I’m proud of you” from Doom would be like “You’re the greatest person I’ve ever met” from Superman.