I love watching the Billy Connolly sketch about the incontinence trousers advert, that has Robbie Coltrane crying with laughter. Billy's description is so vivid and Robbie's reaction makes it twice as funny.
I remember the Billy Connolly one - it was hilarious and he didn't used to f and blind like he does nowadays. I never watch him now - can't stand the foul language. Just goes to show it isn't necessary to make people laugh!