This one is tough because by saying product placement isn't a show also a product? What I'm saying is think Star Wars, Transformers, Pokémon, GiJoe and the likes. They are made to produce a market of consumers of not just the show/story but items made that reference the show like toys. They are cash cows. And literally some shows were/are only originally produced to sell merchandise like Transformers.
So based on that reasoning Stars Wars is king. It sells everything you see on the screen in some form therefore its top product placement. Think books, cards, toys, collectibles, games, costumes, you name it. I also remember at one time Star Wars was indeed the highest movie/show/franchise based merchandise seller all time. Not sure if it still is but it has to still rank top 3. That is what I call great product placement.