I once read a story (I think it was in a sci-fi anthology) about the funniest joke in the world - the problem was that everytime someone worked it out they laughed themselves to death!

My favourite, however, shouldn't hurt too much:-

A man is walking along the road and suddenly his way is blocked by a funeral proccession. At the head are two hearses which are followed by
a man leading a dog, following him a short distance back is a long file of people numbering several hundred. Curious, he asks the man with the dog who's funeral it is. "The first hearse has got my mother-in-law in it - she was a miserable old bag, but the dog bit her and the next day she died". "The next hearse contains my wife - she made a huge fuss and tried to get the dog put down, but it bit her and she died the following day". The first man thought for a while and then asked "Can I borrow your dog for a while ?" "Get in the queue with the rest" it's owner retorted.