Hard to assign the most significant, but:
"Stranger in a Strange Land" by Robert A. Heinlein
Other authors: Tom Robbins, Kurt Vonnegut, Douglas Adams, Isaac Asimov, Harry Harrison, Arthur C. Clarke, Larry Niven, Frank Herbert, Jonathan Kellerman, John Irving, Aleksandr Solzhenitsyn, Charles Dickens, and pretty much all other authors mentioned in other posts here.
While making this list I realized that it probably wasn't any particular book that was stimulating but any stimulating book read when I was young and impressionable.
Movies: "To Kill A Mockingbird". "A Clockwork Orange", "Contact".
Music: "The Rite of Spring" by Igor Stravinsky
Art: "The Garden of Earthly Delights" by Hieronymus Bosch, "The Persistence of Memory" by Salvador Dalí , "The Scream" by Edvard Munch
Originally posted by: KarenMarie
Don't know if I would say it changed my life, but The Long Walk by Stephen King... I found it so friggen disturbing that it haunted me for ages.
QFT - it was unimaginable to me at the time that anyone could be this desparate but some 20 years later have seen several real-life examples. This was almost as disturbing as "Naked Lunch" by William S. Burroughs.