Alien 3
Alien [1979] was brilliant & original. I mean, who'd ever seen an alien-horror flick like
that before?
Then Aliens [1986] was an incredible sequel - equally brilliant & original. Aliens [1986] had me ramped up for the next installment in the Alien series
as much as Empire Strikes Back [1980] did for Star Wars.
But then Alien 3 [1992], not unlike Return of the Jedi [1983], not unlike learning there really was no Santa Clause showed me the magic was gone.
Oh, on its own merits, Alien 3 is probably ok as a movie. But it's never shaken the stigma of being the film that violated me with the knowledge that the Alien franchise was forever done being innovative.
When the two prisoners run into each other & fall down frightened in that "You thought
I was the alien? Ha ha - I thought
you were the alien!" scene
... oh the cinemanity!
(ooo I hope that isn't taken I just thought of it! 
)
Like Yoda with Luke's X-Wing, my hope is that Prometheus [2012] & its remaining installments will elevate the franchise out of the one-note cliche swamp it's sunken into.