I have quite a few fave commercials. of course right now the recent ones i love escape me lol
but when i saw this question i thought of a commercial i have loved for years. it's an old little ceasars pizza commercial for the party pizzas back in the late 80's the whole family is dancing around the table for the "party party" the kid is behind the grandma he isn't paying attention the grandma stops and he walks right into her butt. LOL i mean how many times did that ever happen to you as a kid walkin behind mom or grandma or someone you are looking somewhere else and you are deflected off an ass that stops in front of ya.
ok i found this... this isn't exactly the whole commercial i remember i think the one i saw just had parts of this one in it or something but the butt in the kids face is the same
the worst commercials for me are now all the prescription medication one and all the weight loss commercials
(I will probably update this when i can think of more recent commercials that crack me up)