Jaws.
Making a sequel was completely unnecessary.The chances of the same US town (and family) facing a shark,which again results in multiple deaths,was totally unbelievable.
Beverly Hills Cop.
The original was a predictable and by-the-numbers story,but turned into a classic due to some perfect casting.The story was wrapped up perfectly,and it was fairly doubtful that Axel would socialize with the LA cops in the future,nevermind find himself in Beverly Hills on another crime case.
Psycho
Sequel was pretty decent,but naturally inferior to the landmark classic.Did anybody really believe Norman would be let out,nevermind live in the same house again?
At the end of the day,a shameless cash-in on the lucrative slasher craze,at the expense of a legend.