Anything by Monty Python. They can't be improved on and the dodgy special effects only add to the humor.
2
Labyrinth
3
The Goonies
4
The Princess Bride
5
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
6
Alien (1979). It's still so good 40+ years later
7
Stand By Me That combo of young actors just worked so well, and I just can’t imagine it would work so well again
8
Casablanca
9
Beetlejuice
10
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
11
Airplane!
12
Fight club
13
Shawshank Redemption
14
Galaxy Quest
15
Pulp fiction But luckily I think Quentin Tarantino makes movies no one else would try to remake.
16
Back to the Future. The thought of anyone other than Michael J Fox and Christopher Lloyd playing those roles is blasphemous to me.