The first that comes to mind for me is The Breakfast Club and I hope it never does have a sequel or remake
The Princess Bride is another good one
Edit: another one of my favourites is Wall.E, my favourite Pixar movie. There are also several other Pixar movies that don’t have sequels or remakes yet
2001 A Space Odyssey has a sequel
And it’s actually great! It just exists in the shadow of the original.
Oh, didn’t realize that. BTW I’m adding more movies as I think of them