I've seen a lot of sad sinema it's hard to pick the SADDEST but I think one of the saddest is definitely A.I (artificial intelligence) that was a sad movie. :-(
Many - especially ones in which an animal dies: 1. Where the Red fern, kangaga Grows 2. Marley and Me 3. Old Yeller 4. My Dog Skip 5. The Lion who thought he was People 6. Turner and Hooch