I was talking about this with my father the other day. I can't remember if any of these have already been said.
Texas Chainsaw Massacre 2: The first movie is such a unique movie. It's subtle and creepy and atmospheric. The second movie went the complete opposite direction, with tons of gore, no subtly, and extra comedy. There isn't much the series can do after that without rehashing the first two. I haven't seen the third movie, but I'm told that it's just a typical slasher.
Halloween : As good as 2018 and H20 are, Halloween 1 is a self contained story, and it doesn't need added depth. And from my experience, watching sequels and then rewatching the original kinda changes your perception, and it's hard to watch it with with a fresh mindset. To some it may be ruined. Not to me, but to some I'm sure.
There was more that I can't remember