Worst things for me are not actual ads (although I guess they are in a way) but those stupid little sponsor's idents at the beginning and end of commercial breaks on sponsored programmes on ITV/Channel 4/Sky. I think they're meant to be funny, but they are uniformly pathetic. It almost spoils Shameless for me (fortunately, the programme is good enough to rise above it). A part ends, usually with either a very funny moment or a mini-cliffhanger, and suddenly we're treated to the 118118 goons "hilariously" (yeah, right) acting something out. It only lasts a few seconds but makes me want to break something. If you want to sponsor a programme, fine, but leave the comedy to those who can do it.
Sorry, but I feel better for getting that off me chest. :-)