Late Beethoven works are astonishing. The Great Fugue [1,2] is also remarkable -- fiendishly tough on both audiences and players. It is hard to believe it was written during the early 19th century. The fugue also has an interesting story: originally the 4th movement of a quartet, its terrible reception caused the publisher to request a new 4th movement, and the fugue became a standalone piece. The best description I've heard of it is from Stravinsky: "a contemporary piece of music that will be contemporary forever".