Ah, good 'ol Red Skull. Who else could POSSIBLY rank as Cap's most notorious villain? Beyond just Winghead himself, he ranked as Wizard Magazine's 21st and IGN's 14th greatest comic villains of ALL TIME.
Red Skull, aka Johann Shmidt, is a former Nazi general and Adolph Hitler's confidant. He's currently a Hydra associate bent on world domination and genocide. Years ago, his brain was transferred into the body of a Captain America clone, giving him great power and making him seemingly indestructible.
His first ever appearance WAY back in 1941 in Timely Comics Captain America #1 was written and sketched by the famous Joe Simon and Jack Kirby. In the first few issues, he's revealed to be George Maxon, owner of Maxon aircraft, and he's attempting to overthrow the government by raising money from robbing banks. By Capt. America #7, the Red Skull of today (Johann Schmidt) is introduced, and it's revealed that Maxon was an impostor. He pops up a couple more times in the 50's but then gets his big revival into the silver age of comics in Tales of Suspense #65, where he became a modern continuity character (not just shown through WWII flashbacks).
Most Notorious Act:
Ultimate Comics: Avengers. In ultimate continuity, Red Skull is the illegitimate son of Steve Rogers and Gail Richards who, after his father "died" in WWII, is taken away from his mother and trained on a military base. He has a brilliant tactical mind and physically superior to anything they've ever seen, naturally excelling through the military. Buuut his well adjusted, friendly personality is revealed to be a ruse when he, at 17, takes on his ENTIRE BASE, killing and beheading over 200 men. He then escapes, joins A.I.M., gets his hands on the cosmic cube, uses it to make all of the A.I.M. base members cannibalize each other, he calls the Avengers to show it off, owns them, and finally, Cap arrives and he owns him too. It's a beautifully consolidated arc on what Red Skull is at his core: brilliant, tactical, ruthless, and well... superior.