>>How many films actually get better in the 2nd, 3rd etc<<
Star Trek, the first film was pretty damn good, and topped the series by a longshot - then Wrath of Khan really blew the doors off.
and then, it went kinda downhill, Admittedly.
Wrath of Khan is an alltime fav and classic of mine, not only for the acting, but the real asskick musical score that backed it up so well.
(You ever want Sound Bites in Wav, or the theme in MID format, just howl)
What else ?
Mad Max/The Road Warrior/Thunderdome...Thunderdome was perhaps a little weak on some points, but the story progresses and gets deeper throughout all three, and they do improve along the way.
Escape from New York/Escape from LA.
The exact same movie, twice, more or less...it's a "B" grade movie, but in the context of Carpenter wanting to write a "B" western-style movie, it does exactly what it was intended to...and if you watch both, you'll consider the sequel better.
With the possible exception of 3, it just gets better and better...especially to those with a more complete understanding of the storyline via DarkHorse comix.
(FYI - the now-extremely-rare game Alien vs Predator for Atari Jaguar introduces the idea of Alien vs Predator and touches on why....)
So not all movies go downhill as sequelized, just most of them.