According to the article from The Conversation, Bach's Toccata and Fugue in D Minor held that honor. I'm not one to argue, but if I had to choose, I would have to go to Edvard Grieg's Hall of the Mountain King. I had heard it at an early age, but fell in love with a metal version of it by a band named Savatage in the 80's. Members of Savatage went on to create Trans-Siberian Orchestra.
Another song that strongly reeks of Halloween is Night on Bald Mountain by Modest Mussorgsky. You may have heard it if you've ever watched Disney's Fantasia. It was written as a "musical picture" of a witches sabbath on Bald Mountain during St. John's eve. Very atmospheric!
What music puts you in the mood for the spooky season?