Spider-Man 3.
No matter how bad the dance scenes were or how bad some of the plotting was (the all-knowing butler?), it had moments of true iconic brilliance: The Birth of Venom; The Birth of Sandman; Dark suited Peter blowing up Harry's face; Peter and Harry's Team-Up. It also has the second best set of action scenes in a superhero movie to date (the first being Spider-Man 2).
But most of all despite all the stupid, campy crap that Raimi added--I still cared about these characters. Peter, Harry and even Dunst's MJ were endearing and I still cared what happened to them. I was sad to see Hary die and felt bittersweetness to Peter and MJ's reunion at the end.
X-Men 3 straight-up sucked. Other than Xavier's death, there was nothing memorable about the movie. Despite having nearly twice the budget of X2 and more than double the budget of X1, it had less interesting or intense action sequences than either of Singer's efforts. It was deadly dull. The action was bland CGI mediocrity and worst of all it failed to make me care.
When one factors in that most of the heavy lifting had been done already for fox and Ratner by Bryan Singer...still failing to make me care about the proceedings or characters is a huge testament to the level of failure that movie was. Singer at least made Logan, Rogue, Xavier, Magneto and Mystique engaging and intriguing characters that audiences cared about. Yet Fox/Ratner ****ed it all up and ruined every last one of them.
Plus killing Cyclops, declawing Logan, ruining Magneto and curing Rogue (thereby endorsing "pray the gay away" type logic) is far, FAR WORSE than two dance scenes in Spider-Man 3.
I'm going to stop there.