10. Captain America
Steve Rogers, aka Captain America, is one of the oldest characters created for Marvel Comics. He made his debut back in 1941's Captain America Comics #1, written by Joe Simon and illustrated by Jack Kirby. Captain America was introduced as a patriotic counterpart to the growing tensions overseas, which is why he went on to fight for the Allies during World War II!
Cap spent much of the war kicking Nazi butt, but he got himself frozen for decades following the war. He wasn't unfrozen until The Avengers #4 spelled out his tale in 1964, but you can't keep a good Cap down; he went on to become the leader of the Avengers, Earth's Mightiest Heroes!
Captain America remained a mainstay in Marvel Comics' library for decades, but thanks to some... let's say, not very good movies, he remained a bit obscure to the younger generation of readers until Chris Evans brought him to life for the MCU.
What makes Captain America such an enduring character isn't necessarily his symbology of American patriotism or even how kickass he is on screen, it's his ideals and willingness to always do whatever is right in any situation. That's who Captain America is, always has been, and always will be.