Not to say the sequels are in any way less good, but these are movie series whose first is Art House fodder and after that went to broader, sillier, or otherwise stranger worlds for their sequels
  1. Rocky
    Rocky I is a rather somber, melancholic look at a bum trying to get his life together with a lucky break. It has a bittersweet ending and a terrific performance from Sly. Rocky I-IV are increasingly ridiculous showcases. While they may have merits, no way was Rocky IV going to be nominated for an Oscar
  2. First Blood
    Rocky and Rambo are the hallmarks of this trope, hence why it's called Stalloning. First Blood is a movie about sadness and alienation and prejudicial violence with long, strange sections. Everything after that becomes a ludicrous murderfest.
  3. Magic Mike
    Without Soderbergh and McCaugneghy lending some introspective chops and strange choices, with long takes that emphasize the thoughts rather than the moves of the dancers, XXL became mostly a comedy and bro road trip movie. No knocks, though, it's real fun
  4. Jaws
    The first movie is a master class in minimalist suspense, not to mention a whole new world of camera tricks adding tension. The shark was abstract for 90% of the movie. A decreasing sliding scale of animatronics followed suit and Jaws 2-5 were just about grisly deaths than anything interesting
    All stylish, ambitious, groundbreaking thrillers that were followed by other directors' boilerplate slasher sequels (American Psycho II: All-American Girl, Henry 2: Mask of Sanity and Halloween II)
    Suggested by @angusisley