OK, this might not be the most accomplished piece of literature ever produced, but for sheer fun and bombast, the Hitchhiker series can't be beat. Adams invests the books with a contagious sense of fun, tempered by a deep cynicism about the state of the universe as a whole. So on the one hand, you have an alien choosing the name "Ford Prefect" to try to fit in with Earth's dominant species (the car); and on the other, the total annihilation of our planet to make way for an interstellar bypass. Both are funny; one is cuttingly so.
One of the weirder books on this list, Shadow reads more like a fantasy, exploring a world of mouldering cities and societal factions with an archaic vocabulary that can be tough to understand at times (and the author deliberately omits a glossary, the bastard). But if you stick with it, you'll find this is one of the most intriguing sci-fi series of the last several decades. Secrets of ancient technology are hinted at throughout the beginnings of Torturer (the first in a four-book cycle), leading to the reader's gradual realization that this is not a Tolkien-esque fantasyland but a far-flung future. Modern culture is a buried legend in this world, and yet bits of advanced technology survive, creating a strange world where robots and magic coexist. Be warned, though: If you pick up the first book, you'll need to read all four for a satisfying ending.