I'm going to say Til Schweiger. Second choice, Viggo Mortensen. I know a lot of people are calling for Christoph Waltz or Mads Mikkelsen, but I think Waltz is too goofy and kind of effeminate, which worked EXTREMELY well for Hans Landa, but Red Skull is a different breed. I don't think it's sound logically to say "He played a badass Nazi villain before, he can do it again!" As for Mikkelsen, I just never thought he had much screen presence. He's kind of slight.
I think that Red Skull should be a very intense, intimidating, scary, violent, serious character. I think Red Skull should be old enough to carry authority, but young enough to be a physical threat, which is why I think the late 40s, early 50s age range works.