Most people are familiar with the Pink Panther movie series:
The Pink Panther (1963)
A Shot in the Dark (1964)
The Return of the Pink Panther (1975)
The Pink Panther Strikes Again (1976)
Revenge of the Pink Panther (1978)
Some of those people know they were a collaboration of sorts between Peter Sellers and director/writer/producer Blake Edwards (you could throw in Henry Mancini as well).
Fewer people, particularly those born after 1970, have heard of the trio's movie The Party. It came out on April 4, 1968, and according to IMDb, did fairly well at the box office. Overall, it's fairly mediocre. The movie is basically set the entire time at a party. Duh. But this is exactly what I find appealing about it. You can sort of imagine you are there, at the party. And what a party! I love the mid-60s architecture, furniture, interior design, clothes, colors, music, etc. It's very 1968.
The Party isn't the greatest movie ever made, that's for sure. But, there are scenes with Peter Sellers that are so hilarious, it should be considered a classic.