Network - a timeless classic, every scene, every line has weight and meaning. Incredible acting. Played my DVD of it recently and was amazed how it still holds up.
Dr. Strangelove - another movie that just gets better with age.
The Godfather Parts 1 & 2 - 'nuff said!
Porky's - juvenile, inmature, childish and just plain fun!
Animal House - see Porky's.
High Plains Drifter - My favorite western.
The Lord Of The Rings Trilogy - epic filmaking, I never thought they would make as good as they turned out.
Forbiden Planet - 1950s sci-fi that still holds up. Check out a young Leslie Nielsen

Best regards, Ralph