6. Red Skull
Captain America has been battling Johann Shmidt - AKA the Red Skull - since World War II. Born nearly a century ago, you might wonder how Skull hasn't died.
Well, he has. A lot.
After he perished from old age, his bio-chemist, Arnim Zola, created a clone of Captain America, and inserted Skull's mind into it.
When he was assassinated by the Winter Soldier, he transferred his mind into his rival, Aleksander Lukin. When Lukin was killed, Zola inserted Skull's mind into an android body. Shortly after, he tried to insert his mind into the body of Captain America but this plan failed, seemingly destroying Skull's consciousness.
But Skull had a back-up plan. (In case you can't tell, he has a lot of them.) Skull has a series of clones hidden around the world that will activate at certain times throughout history. When Skull died, one of these clones awakened, and stole the brain of Professor X to give himself psychic powers. This clone was ultimately killed by a brainwashed Hydra version of Captain America (it's a long story).
Recently, Skull has been resurrected by the vampiric Selene, but it won't be long before he bites the dust yet again.