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I like watching old British films mainly from the 1950s, 1960s and 1970's. There are a lot of these type of films on the Talking Pictures TV channel.
Even if the film is not brilliant, the viewer can see social history (how people lived etc) in these past times:
Some of my favourites include:
Bedazzled (1967)
The Bargee
The System (1964)
The Knack ...and How to Get It (1965)
I also find it difficult to name one film.
But Sleuth with Lawrence Olivier and Michael Caine is very satisfying and holds the attention. It is one of only two films where the full cast were nominated for best actor oscar - it only had two people in it. Of more modern films I enjoyed 'Hugo'. An older film I like is 'The day the earth caught fire', the conversation in that film is like in no other, a black and white British film of the early sixties it knocks other disaster movies for six. 'Paths of Glory' an early Stanley Kubrick film is thought provoking and another Kubrick film 'Barry Lyndon' has some of the most beautiful photography in a film ever.