I love this one featuring the late great Spike Milligan as it's very silly and always made me laugh
http://uk.youtube.com/watch?v=YPVyMJIojro
But best one, and I can't find it online, was the Rutger Hauer Guiness one where he is sitting in a bar and says 'If you really concentrate you can tell what I am thinking' a seperate identical advert was occasionally shown late at night with a Rutger Hauers voice saying ' My teddy bear's name is Edward' . The next time you saw the advert you'd be sitting there saying ' My teddy bear's name is Edward' and everyone else would look mystified if they hadn't seen the ad and if they had they agreed with you, absolute genius. If I find it I'll post it.