UB makes a good point about humor being an individual thing. I never cared for the Three Stooges, but the Marx Brothers crack me up. An alien visiting earth would have trouble differentiating the two.
So with that caveat, I'll say that I am a fan of Terry Pratchett's novels. Not everybody to whom I've recommended them has appreciated their sly and bizarre humor. Start with
The Color of Magic or
Going Postal (both of which have been made into movies, which, along with
HogFather, didn't rise to the novels' quality).