I’ll second Guards, Guards! It’s such a funny book, and it introduces characters that go on to grow and change a great deal. Pratchett gets both the funny and the humanity.

I’d also recommend Thief of Time as one of Pratchett’s more standalone books. It does have recurring characters from elsewhere in Discworld, but it is more self contained and one of my personal favorites.

I read Slaugterhouse Five once every couple of years. It’s not my personal favorite Vonnegut novel, but I think its probably the most re-readable. It’s also a novel that I get something new from at different stages of my life.