Terry Pratchet is great, but check out some of his early sci-fi stuff. 'Strata' has a great idea, that various sentient beings have built the planets and layed down the fossils as a joke, right back to...well, I'll let you read it.
Douglas Adams, Tom Sharpe was funny in my teens, as was Ben Elton back in the day.
Currently, Dickens is quite funny if you can get past the paragraphs that go on for half a page, but he was being payed by the word. His observations still stand up today.
Dilbert is great, I heard Scott Adams just get's emailed ideas now from people in 'the cubes' based on their real life.
Dave Gorman and Danny Wallace can be good, but I always have the feeling that they put themselves in slightly contrived situations. Having said that, See Gorman ranting aboout his tatoo in "google whack" and you can see that he really puts his all into it.