It would be a long, long list. Here are a few:
Gone with the Wind—always good for another viewing
Bonnie and Clyde—fresh in my mind since I saw it again last week for the first time since its first run in theaters
Pygmalion (with Leslie Howard and Wendy Hiller)—tops
Anything by Alfred Hitchcock, one of the all-time greats
Look up the big stars from the past—the likes of Bette Davis, Cary Grant, Katharine Hepburn, and Humphrey Bogart—and pick out a few of their pictures.
Check the Oscar winners from the forties and fifties and sixties and sample them.
Find interviewers with directors whose work you enjoy today and see how they answer the question about whose past work influenced them.
Sign up with Netflix, give 5 stars to the pictures you like best among the oldies, and see what others of the same sort Netflix recommends.
[Edit] Sorry, my answer is very U.S.-oriented, and I see that you are in the U.K. There is a great tradition of wonderful British pictures. Here’s a starter list if you like the comedies.