7. Loki
FIRST APPEARANCE: VENUS #6 (1949) Based on the God of Mischief from Norse mythology, Loki is the adoptive brother of Thor, and a member of the Frost Giants, possessing similar physical advantages to the Asgardians who he is so often a bother (though generally being able to withstand more damage than most Asgardians). Not only is Loki extremely intelligent, but he has an expert knowledge of magic, and is able to cast illusions that are capable of projecting over city-wide distances. His hypnotic abilities allow him to influence others, and he can even perform astral projection. Loki can also shape-shift and assume the physical likeness of others, be it a human or an animal, though usually cannot assume the superpowers of whatever form he takes. Though all of this, combined with his clever scheme to cheat death, makes him a tremendous threat, he is often brought to his knees by way of his own arrogance, which causes him to over-estimate his abilities, and come up on the wrong end of Thor's hammer. He was played brilliantly by Tom Hiddleston in Thor and The Avengers.
6. Ra's al Ghul
FIRST APPEARANCE: BATMAN #232 (1971) Ra's al Ghul is a master criminal and the leader of the League of Shadows, who believes in restoring balance to the world by tearing civilisations down when they become too decadent. He usually attempts to eradicate humanity through large-scale terrorist attacks, and his use of Lazarus Pits - pits containing chemicals which can return the dead to life - has allowed him to leave for centuries. He has tremendous respect for Batman to the point that, despite figuring out his identity, he refused to divulge it, and had initially groomed him to take over his empire. Bats' usually unambiguous quest for justice is muddied slightly by his on-off relationship with Ra's' daughter, Talia, as if Ra's' potentially never-ending lifespan wasn't bad enough (though excessive use of the Pits will cause his mind to warp). Ra's was played incredibly by Liam Neeson in Batman Begins, and briefly during a dream in The Dark Knight Rises.
5. Venom
FIRST APPEARANCE: THE AMAZING SPIDER-MAN #252 (1984) Venom is often referred to as "the Symbiote", for it is a sentient alien creature that requires a human host in order to survive, and upon finding one, will bestow said host with an array of superhuman abilities. Though the Symbiote originally bonded with Spider-Man himself, the most famous host is unquestionably Eddie Brock, who became Venom, easily Spider-Man's most beloved and intimidating opponent. Though Venom typically has a black, muscular appearance, the Symbiote can be concealed entirely in order to escape detection, while the "suit" contains a dimensional rift that allows the wearer to carry objects with them that won't protrude outside the bounds of it. Because of its prior contact with Spidey, the Symbiote allows any future wearer all of Spider-Man's powers, while also being undetectable by his Spidey sense. Venom's major weaknesses are fire and sonic waves, which cause the symbiote great distress and may potentially force it to exit its host in search of another. Venom was portrayed with a certain degree of controversy by Topher Grace in Sam Raimi's underwhelming Spider-Man 3.