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Mentioned it before, but Pirates of the Caribbean 2 is one of the worst.
It undid the happy ending of the first film - and by the end of the 3rd film Will gets screwed over and is cursed to only see his wife and child once every 10 years. And while I'm not really into "happily ever after" endings, it definitely fit a lighthearted adventure film like Pirates, and there wasn't any benefit plot-wise to having the 3rd film end the way it did.
It brought back the villain (Barbossa) from the first film, and ruined him by turning him into a likable character without any reason given for his 'change in heart'. They even had him be the one to marry Will and Elizabeth in the 3rd film - why would Will or Elizabeth want to be 'married off' by the guy who'd previously contemplated throwing her to his crew to be raped - just because he decides he's a "nice guy" all of a sudden?
It portrayed Jack as a much more cowardly character. He drags his crew all around the seas endangering their lives just to save his own ass, and he even contemplates abandoning them when the Pearl is attacked by the Kraken (though he changes his mind). In the first film he was a rougish character, but still honorable beneath his rough exterior.
It took minor, unremarkable characters (like Norrington) and tired to shoehorn them into major plot roles.
And most of all, it took a great film and ended up turning the series into a bunch of mediocre, GCI-laden cash grabs with Johnny Depp being the only entertaining part of the films.