It would be a very long list as I like many. In no particular order just how they come into my head here are some of my favorites:
Since you said "classic" I expanded a bit to some later films but still think that the later ones expanded the craft.
Psycho with Tippi Hedren
Seven Year Itch with Marilyn Monroe naturally
How to Steal a Million with Audrey Hepburn
"Any" Jack Lemmon movie including The Outsiders (not that Steve martin thing) and Prisoner of 2nd Avenue.
The original 1930 version of "All Quiet on the Western Front"
It's a Mad Mad Mad Mad World from 1963
The effects of Gamma Rays on Man in the Moon Marigolds 1970
(with Joanne Woodward. won academy award but impossible to find on video. AMC runs it once in awhile and I taped it there. )
The Good, the Bad and the Ugly with Clint Eastwood
Sunset Boulevard with Gloria Swanson
Une Femme est Une Femme (famous French Film from 1961)
Also "Breathless" (the french movie from the early 60's as well
Hiroshima mon Amour ground breaking French film from the 50's.
Blowup incredibly colorful "Mod" film from London in the 60's.
Read My Lips also french film
Contempt with Brigette Bardot and Jack Palance
No Down Payment 1957 best movie ever filmed about what surburban life was like in Southern California in the 50's when ti was developing. Tony Randall, Joanne Woodward and lots of other well known stars.
Fool for Love with Sam Shephard and Kim Bassinger
Barabas, which actually filmed the scene of Jesus' crucifiction during a very rare total solar eclipse.
How the West was Won! Filmed in Cinerama I was lucky enough to see this one in the theatre as a child when first came out. For folks who never got to see in "Cinerama" that was a process similar to Cinemascope widescreen, but used three projectors running three different rolls of film shot with three separate lenses to give an ultra wide angle shot. Cinemascope, which most of ther wide screen movies used is different as used three folls of film in one camera with one single lens. Incredible to see on the big screen. You can see the effect of this in the wide open shots in the movie as the far off background doesn't totally line up in two points due to the three lenses having been "scrunched" to fit a tv screen, or even a modern "wide" format which is still not as wide as Cinerama originally was. Panavision like they usually use nowdays is one big lens and one roll of film.
That's too many already and could go on all night. Leave the rest for others to suggest. :-)