Marilyn Monroe's best film is probably The Misfits.
If you like crime/noir films check out Double Indemnity, The Postman always rings twice (not the 80's Jack Nicholson version), Night and the city (not the 80's Robert De Niri version), Laura, Mildred Pierce, White Heat, The Roaring Twenties, The Asphalt Jungle, The Killing (Stanley Kubrick's first film), Naked City, Shock Corridor, Brighton Rock, I am a fugitive on a chain gang, The Petrified Fortest (early Humphrey Bogart when he always played villains - great film), The Big Sleep, Farewell my lovely......and many more.
Also, any early Hitchcock film is worth watching, but in particular The Lady Vanishes, the 39 Steps, Sabotage, Foreign Correspondent, Shadow of a doubt, Notorious and the Man who knew too much (1934 original, not 1956 Jimmy Stewart remake). Actually, I think Notorious and Foreign Correspondent just might be my favourite Hitchcock films.