Anything by Monty Python. They can't be improved on and the dodgy special effects only add to the humor.
Labyrinth
The Goonies
The Princess Bride
Mr. Bean. No one will ever be able to recreate the character as well as Rowan Atkinson. Really anything from Rowan Atkinson can never be remade. Blackadder for instance
Alien (1979). It's still so good 40+ years later
Stand By Me That combo of young actors just worked so well, and I just can’t imagine it would work so well again
Casablanca
Beetlejuice
Besides the fact that Tom Hanks was perfect, I don’t see a modern remake of Forrest Gump being received well based on cultural shifts alone
Airplane!
Fight club
Shawshank Redemption
Galaxy Quest
Pulp fiction But luckily I think Quentin Tarantino makes movies no one else would try to remake.
Back to the Future. The thought of anyone other than Michael J Fox and Christopher Lloyd playing those roles is blasphemous to me.