Three classics here that I think a few people could agree on.
1) Guiness - horses. I think it was voted best advert ever on a TV show a few years back.
2) Sony Bravia - bouncy balls. I saw a 'Making Of' documentary and they're quite loud and lethal. And it was over so fast.
3) Tango Clear - Sony Bravia spoof. This controversial one was an obvious p!ss-take of the above ad and reportedly had complaints from Swansea locals who claimed they didn't give the crew permission to shoot, that too much damage was done, that no one cleared the mess afterwards and that the toad was hurt during filming.