Enough about movies that don't need sequels, which films benefit from them?
Child's Play: While the first one is well made with top notch special effects, it \*is\* a pretty standard murder mystery throughout most of the runtime. Not only do the sequels give Brad Douriff time to really show off as Chucky, but they also manage to experiment with tones, characters, and concepts while all still being set in a single coherent continuity (which isn't an easy task to say the least...)
Friday the 13th: The 1980 great is pretty solid in a vacuum albeit basic (~and too slow for some people's likings~) but some of the later films just blow it out of the water, having something for everyone to enjoy and staying good for AT LEAST 6 to 7 installments before falling off. Also, do we really need mention Jason's status as a pop culture icon? Not only is he basically THE slasher villain but try and find someone who associates those old hockey masks with the actual sport instead of him...
Godzilla: There's no denying that the original film is an important classic in terms of both themes and practical effects, but let's be honest, what's really made the character endure the test of time is his extensive rouges gallery and mythos crafted over the course of 71 (going on 72) years! Not only that but also his surprising versatility, being able to represent various concepts (from postwar trauma to a government's ineffectiveness against natural disasters) as well as adaptability to different tones. It also might be one of the few ongoing franchises that isn't met with complete apathy.
NOTE: I'm not trying to bash certain movies (I actually enjoy the first entries from the series I've mentioned) in nor am I claiming that my opinions are the end all/be all, this is just a fun discussion topic just to go against the grain from the usual posts you'd see on here or elsewhere.