You can afford to be brutally honest here, because the question is about which product/service you most like. It begs the question that you do actually know about the firm and indeed have a preference. For any marketing interview you must prepare in advance by familiarizing yourself with what the company does!

A successful product/service is worth concentrating on. This will allow you to identify its appeal and then describe how the marketing strategy has worked. An example might be Walker's crisps tasting nice, but through the marketing strategy attached to them they are now also perceived as funny and good fun. This is obviously important when one of their biggest markets is children.