I can't speak for the other films on your list as I've not seen them but if you think 'It Follows' deserves to be thought of in the same way as a Nightmare on Elm Street or The Thing or Alien then we are definitely not going to be on the same page. It follows had that slightly glazed netflix sheen and it was watchable, which puts it FAR above any of the rest of the trash tier shovelware netflix puts out these days, but wow, it's not Alien and that has to be the bar we are setting if you're talking "scariest movie".
Insidious is a good film and genuinely unsettling, much better than It Follows for sure but I doubt anyone will be thinking about it in 20 years time. It's maybe on a par with "The Blob" or something.
Are we talking about the same It Follows? It wasn't a Netflix film... But anyway, can't say I agree with what you're saying.
I said it's a great modern horror, not that it's better than Alien or The Thing. It's well made, well acted, good storyline with an interesting theme and a very high concept. The final point making it a film that will pass the test of time. Whilst I like Alien and The Thing more (two of my favourite ever films), I found It Follows far more unsettling. Different people find fear in different things. I will never find any movie about aliens or space scary simply because my mind cannot let go of the fact that this is not a situation I would ever be in and also, I struggle with finding aliens scary because this is not what I actually expect an alien (if it exists) to be like.
Insidious is great and I think you're under-selling how people will remember it in time. It was the first (I think?) horror film about astral projection (as far as I'm aware) and has a bunch of potentially iconic moments/scares. The face behind Patrick Wilson during that exposition scene is on many lists as one of the best jump scare moments ever. It also re-launched James Wan and Leigh Whannell's careers, who both have gone on to become big filmmakers (especially Wan) and it also kick-started the resurgence of the haunted house sub-genre for a good while.
Not saying either of these two are as good as Alien or The Thing but just pointing out that there are many good horror films being made out there.