Not sure what my pick would be – maybe “So it goes” from Slaughterhouse Five as a perfect illustration of how shit happens and then we die?
Or on a more optimistic note: ““And I urge you to please notice when you are happy, and exclaim or murmur or think at some point, ‘If this isn’t nice, I don’t know what is.””
Actually, pretty much any quote from Vonnegut has potential to be life changing or mind altering.