Without having thought much enough about it, my hunch is that the first movie works best not merely because of the typical 'first one is frech, second and third one are lame copies' effect but because the characters are interesting (OK, Goldblum's funky mathematician and Attenborough' naive park-owner are utter clichées but gee, I love them) and because the movie features Spielberg's typical absent father motive we have known since the days of ET.
The basic idea is that that ET has to come down because the father is absent and once this issue is solved (Isn't there a scientist who plays a substitute dad or something like that?) ET can leave again respectively that the entire adventure with the dinosaurs is merely there in order to make Sam Neill's character discover his parental instincts.
The scenes where Neill's character protects the two kids and travels with them through the park are for me the best ones which is why Spielberg's obsessive absent father motif isn't just some stupid theme you can talk about afterwards but actually the ingredient makes the movie into more than just a movie about dinosaurs. Not that there is something wrong with that.