10. Spider-Man 3
I believe there are people still trying to work out the plot to Spider-Man 3 in a laboratory in the middle of nowhere because us normal folk didn't understand a word of it as we were too busy being perplexed by Peter Parker's emo turn and cringy dancing. There are villains coming from all over the place and none of them are given central focus, which leads to a rambled attempt at a plot as lots of people scowl and make lots of noise for reasons we're very confused by.
Venom, Sandman and Harry Osbourn are the villains, but none of them really work as they're more interested in battling each other for screen-time than Spider-Man. The script somehow makes Sandman responsible for the death of Uncle Ben and Venom's inclusion feels forced and contrived. The villain should have been Harry, and Harry alone, as that builds on the last film and has a nice personal touch, but the film is all over the place and has virtually no substance.
Spider-Man 3 was one of the most highly anticipated films ever, but Sam Raimi's confused mess of a movie was a massive let down and millions of fanboys had to cry themselves to sleep as a result.