Un Chien Andalou is the first film that really got me interested in surrealist cinema. While not the first surrealist film (that goes to The Seashell and the Clergyman by Germaine Dulac), it is particularly notable due to Salvador Dali’s involvement.
The film, directed by Luis Buñuel and Salvador Dali, has minimal plot in which the characters exist in a dreamlike world. It was Buñuel’s directorial debut.
If you’d like to explore surrealism in cinema further, Maya Deren’s Meshes of the Afternoon is another favorite of mine.