Wow, people are throwing out movies left and right.. of course 99% of all movies will never be remade. The more appropriate question is what FRANCHISE movies will never be remade.
pumpkinhead critters garbage pail kids the movie leprechaun howard the duck baby mannequin one that i would like to see remade would be krull... in a lotr kinda way.
Pretty much any Steven Spielberg movie. North Dallas Forty Pretty much any Hulk Hogan movie Rudy (that would be sacrilege) The Godfather and The Godfather: Part II Forest Gump
You can't improve perfection. I would have included Blade Runner, but there has already been about four different versions of the slice of my life covered by the flick.
I was going to say the "original" Star Wars Trilogy (Episodes IV-VI), but they'll probably be remade in some animation format (a la The Clone Wars) at some point. I'll go with: Repo Man Cannibal: The Musical The Big Lebowski Movie that should be remade (even though the original was awesome): Looker