The first Jurassic Park movie is all time one of the greatest films ever made with the special effects still holding up to this day. The 2nd film was still very enjoyable in my opinion but it was just a cookie cutter sequel not bad, not good. The 3rd film wasn’t great at all. But compared to the rest of the series the 3rd film is basically the godfather.
As another personal pick the 1st blade movie is a hood classic good. The other 2 not so much.
Terminator? T2 is regarded as that franchise's best flick and it's only downhill from T3 onward.
Robocop as a franchise also infamously went down the toilet after the first movie, each sequel degraded in quality until the complete mess that was Robocop 3 happened.
Also Alien and Aliens were good, the rest not so much.