4. Venom - Spider-Man 3
This is the franchise I mentioned in the introduction with two entries. Not The Avengers (not many of their villains are iconic) and surprisingly not Batman (despite what the late nineties did). The other villain is one you wouldn't immediately think of, but for now I'm going to contend myself with the incredibly obvious choice. Venom is the quintessential fan favourite. Some like Doc Ock, some like Green Goblin, but everybody loves Venom. Eddie Brock is part antithesis, part rival to Peter who transforms into the same for Spider-Man. After a dark side of Peter has been exposed internally by the symbionite we get a literal showdown through Venom. Hes a relatively late addition to the Spider-Man villain canon, coming over two decades after that fateful spider bite. Sam Raimi never wanted Venom in the film, originally wanting to explore Peters dark side through Sandman and his conflict with Harry, but Sony execs forced his hand. The original two storylines became incredibly weak with little screentime and the Venom plot bored fans thanks to its complete mishandling; "Stop peeling you're face back. We came to see Venom, not Topher Grace." Theres been long standing rumours of a return to the character, from part of the alien left at the end of Spider-Man 3, talk of a stand alone movie and the distinct possibility of an appearance in one of the upcoming Amazing films, so hopefully therell be a better second attempt.