Slaughterhouse-Five is a pretty good starting place just because it introduces a lot of thematic elements (flash-forwards, flash-backs, Kilgore Trout, cruelty of humans, etc) that run through Vonnegut's works. Like kilgore said, there really isn't too much importance on order, except Timequake has a more meaning the more Vonnegut you know. But even then you can enjoy it on its own. Mother Night is great though. Definitely one of my favorite Vonnegut books and I've read almost all of his stuff.