Accept the past. Live for the present. Look forward to the future. Robot Fusion
"As our island of knowledge grows, so does the shore of our ignorance." John Wheeler
"[A] scientist looking at nonscientific problems is just as dumb as the next guy." Richard Feynman
"[P]etabytes of  data is not the same thing as understanding emergent mechanisms and structures." Jim Crutchfield
I think Slaughterhouse is overrated because having a moral anti-war lesson makes things popular, and gets them assigned in schools such that it's the only one a lot of people have read. Though if you're religious you might prefer it to those that make fun of religion.
Having read all of them, I rank them in this order (recycling an old blog entry here to save writing time):
1. The Sirens of Titan - The best book I’ve ever read, and the only one I’ve re-read many times. Malachi Constant’s journey is one of illusion-shattering, going from being a rich guy on top of the world to a mere tool used by others. He’s pathetic, like all of us. He learns to accept things.
2. Cat’s Cradle - The seminal work of Bokononism, and also (as the final sentence of the Books of Bokonon puts it) a history of human stupidity. Impressive and perhaps his most thought-provoking, but I don't think it has quite the flow of Sirens. Bokonon’s 58th calypso:
I wanted all things
To seem to make some sense,
So we could all be happy, yes,
Instead of tense.
And I made up lies
So that they all fit nice,
And I made this sad world
3. Bluebeard - The story of Rabo Karabekian, an abstract impressionist whose life too closely resembles his art. No, I don’t like art, but it’s a great story. Probably the most character-focused.
4. Slaughterhouse Five - You don’t have to become unstuck in time to feel like you’re watching random bits of your life from the outside, nor to learn how to accept it. Slaughterhouse was the first of his books I read, because it was assigned in high school.
5. God Bless You, Mr. Rosewater - A story of rich and poor.
6. Galapagos - A million years from now, the human race has evolved into a bunch of brainless creatures flopping about and having sex.
7. Mother Night - An American in Germany goes to work for the Nazis sending out coded messages to the allies in his propaganda broadcasts. He spends the rest of his life dealing with the ghosts of his past, wondering if he became who he pretended to be. Good, but the movie is better than the book.
8. Timequake - A timequake forces the 90’s to repeat. His last novel, it’s kind of the retirement home of his characters.
9. Breakfast of Champions - An outwardly happy yet inwardly suicidal used car salesman can’t stop people from trusting him and believing that his act is real. It’s pretty much about the problem of being liked for who people think you are, but not having an identity on the inside. Vonnegut’s mid-life crisis, filled with bad drawings and childishness. Good, but not a good starting point, requires familiarity.
10. Hocus Pocus
12. Deadeye Dick
13. Slapstick - Potentially offensive postapocalyptic tale, not a good starting point.
14. Player Piano - His first novel. It’s straight sci-fi, not really funny, doesn’t have the Vonnegut feel. Not that it’s bad, it isn’t.
Note that Welcome to the Monkey House and Palm Sunday are just collections, not novels -- like Fates Worse than Death, Bogombo Snuff Box and A Man Without a Country. Personally I like his novels better than the collections. Happy Birthday, Wanda June is a play, not a novel, and so doesn't make as good of reading.