Now, I love to rewatch movies over and over again, and re-reading books endlessly is how I justify my big ol' library. (Just the fiction alone you can see listed at LibraryThing under "KCozy"). But certain things leave their mark on you the first time in a way that can never be recaptured. Here's a few of mine (but ask on a different day and you may get different answers).
Poem: T. S. Eliot's The Hollow Men. Read it in high school and had such an epiphany. One of those "This is IT!" moments.
Movies: Oh, so many to choose from, but two that stand out are Terry Gilliam's Brazil, which literally left me breathless, or Peter Weir's The Last Wave, which I saw at an early age and which warped my brain forever.
Novel: Again, it's hard to narrow the choices down. For fantasy, C. S. Lewis' The Chronicles of Narnia is one choice, though I have to give a shout-out to the underrated Chronicles of Prydain by Lloyd Alexander. Margaret Atwood's Cat's Eye gave me a shock of recognition for its portrayal of Mean Girls and the damage they can do. For horror, it's Stephen King's The Shining; for mystery, James Ellroy's L. A. Confidential; for dystopian fiction, it's George Orwell's 1984. For short stories, Ray Bradbury's "The Aqueduct" literally made me scream the first time I read it.
TV: Any number of Twilight Zone episodes spring to mind. But what I'd most love to re-experience is the first episode of Blackadder Season 2. The first minute in this clip made me laugh so hard I nearly hurt myself, the first time I saw it.