Marilyn Monroe flaunting her chest in Some Like It Hot (1959).
That is one of the greatest movie comedy of all time. Both Tony Curtis and Jack Lemon were hilarious, and Marilyn Monroe was definitely the main star of the movie. 
It's a classic for sure.
At least on some of the posters for the movie it reads: “Marilyn Monroe and her bosom companions Tony Curtis & Jack Lemmon”. So Marilyn, and emphatically even her bust, is mentioned before the male actors. With that kind of advertising, the movie wouldn't deliver if Marilyn's breasts weren't on display in the film. And within some limits they certainly are - in a quite risque fashion for the time. She wears costumes that show her nipples poking through, other costumes where the chest area is of see-through fabric with only her nipples and areolae covered. And sometimes when she wears less revealing clothes, she dances and plays ukulele in a way that is surely meant to show her breasts jiggle and wobble.
I would compare Some Like It Hot to many of Dolly Parton's films in how much effort has gone to presenting the main star's breasts to the audience. Alas Marilyn doesn't have Dolly's talent upfront, so the visuals provided aren't as ...uhm ...indubitably breast fetishistic as in Dolly's movies.
One thing I find adorable and unusual about Marilyn's acting is how she uses her bosom to express emotion and intent. Like in the gif below (from the same movie).