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.
Both of those make a strong case for the second one being better than the first.
But yeah, they definitely drop off a cliff.
Alien and Aliens were different types of movies. Horror vs Action. That's why Aliens works. It isn't a rehash of the first.
T2 was similar to T1, but a change in dynamics like Aliens did. Sarah Connor was more in charge and knew what to do about Terminators by the sequel.