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.
I'm just thinking in big franchises, because I'm not a well-schooled film-nerd. But even I can think of a lot of examples of sequels better than the originals.
It's a matter of taste, but I think Rocky 2 is the strongest film in the franchise. I think it's more commonly believed that Aliens is a stronger film than Alien. Evil Dead 2 is better than Evil Dead, but that might not count since its a soft reboot. Star Trek 2: Wrath of Khan is pretty much a classic example. Terminator 2 is a very different movie than Terminator 1, making comparison weird but it's usually regarded as better. Sam Raimi's Spider-Man 2 is arguably stronger than Spider-Man, though obviously 3 was a bit of a mess.
Going to put this in a separate section. Admittedly, whether this should count is arguable. Still, If you count Marvel movies as direct sequels to others, it gets ridiculously easy: Thor: Ragnarok is so much better than all the other Thor movies its kind of nuts. Winter Soldier is stronger than First Avenger and Civil War might be better than both. Guardians of the Galaxy 2 is the strongest film in that franchise.