I really loved Kurt when I was in college. Ate it up. Of all of them I think I like (and I'm sure I'll be the only one here) I like Wampeters, Foma and Granfalloons the most and still read that. Though Cat's Cradle was a critical touchstone in my understanding of both the promise and damnation of science and technology, as well as doing such an excellent job of point out the basic immorality of both on their own (without control or oversight).
But it was Slaughterhouse 5 that really rocked my world when I first read it my senior year in HS. It's on a very short list of stuff that I've read that really changed my views, my outlook and my life.1 It solidified my anti-war views as well as my 'secular humanist' outlook and touched something very deep in me. It shook me all night long.
I suppose I should go back and re-read some of them, particularly Sirens of Titan, which I don't think I got when I read it.
1. Dorothy Day, The Long Loneliness, Thomas Merton, The Seven Story Mountain, Herman Hess, Siddhartha, Charles and Mary Bead, An Economic Interpretation of the Constitution, Barbra Tucuman's The Pursuit of Folly and August 1914, and Eric Hoffer's two books The True Believer and The Ordeal of Change along with the Tao Te Ching and the Karma Sutra are the rest of that short list - if you wanted to know.