For me, it's films where the casting and directing is irreplaceable or they're set during a period in time which would be near imposable to recreate.
To Kill a Mockingbird
The Quiet Man
The Wizard of Oz
Die Hard
Groundhog Day
The Princess Bride
The Man Who Shot Liberty Valance
Rocky
Planes, Trains & Automobiles
Uncle Buck
Stripes
Caddyshack
Quigley Down Under
High Plains Drifter
Pale Rider
Back to the Future: trilogy
It's a Mad, Mad, Mad, Mad World
Lethal Weapon