After the Red Skull, I don't think you can name a bigger enemy of Captain America than Baron Zemo. He's been the source of multiple major Avengers and Cap plots (to say nothing of launching The Thunderbolts).

Thing is, why Zemo? Back in 1963, he had nothing to offer The Skull couldn't provide. You could have swapped them seamlessly.

Kirby created Zemo. Kirby was also part of the team (with Simon) that created Red Skull. Was there something that prevented giving The Skull Zemo's role in 1963?