I've seen the Cadbury ad with the gorilla playing the drums a few times, and every time, I think it's funny (albeit bizarre).
I agree that the RSPCA ads are very effective. They always bring me to tears.
I used to like the recent John McEnroe vs. Bjorn Borg Tesco commercial, but they've aired it so often that it's lost its appeal.
I really can't stand the M & S commercials with the woman describing food in a sexy voice. Who is she--Nigella Lawson?
Also, the Halifax commercials are getting pretty annoying.
I can't stand any car insurance commercials--particularly Sheila's Wheels.
I'm sure there are others, but they don't immediately leap to mind.