Spiegel im Spiegel (lit. 'mirror(s) in the mirror') is a work for violin and piano from 1978 by Estonian composer Arvo Pärt (1935). He composed it just before his departure from Estonia (then part of the Soviet Union) to Austria, and later Germany. This minimalist piece is one of the shortest in my list, and also one of the most controversial. I've seen classical music lovers deride it as a snooze fest, or 'boring as hell', while others (including me) fall for its hypnotic qualities. It has certainly reached a wide audience, having been used in dozens of movies and TV series. The average playing time is 9 minutes (AllMusic). My version is by Tasmin Little and Martin Roscoe on an EMI CD.
