For me it's ones featuring eating/drinking noises. Like many other people I can't bear the sound of eating/drinking/mouth noises (I actually hadn't realised how common it is until I read several threads on mumsnet about it).
There are a couple of current/very recent adverts that prominently feature these kinds of sounds - a Sainsbury's one with loads of closeups of people eating, one for tea with a toy monkey doing a loud slurp and "ahhh" and a little while back there was a radio advert for weightwatchers that was a woman talking through mouthfuls of food. That was the worst. (I know I'm being U really because most people are fine with it but ARRGHH)
Which ones can't you stand?