No. 1: "Transformers: Revenge of the Fallen" -- $402 Million
This debacle made a cool $400 million. So apparently some worthwhile purpose was being served by it; it's just that being a good movie wasn't one of them.
The film's attempts at story and character are drowned within a blender of metallic action. As is commonly the complaint of big summer blockbusters, it had few endearing human elements. But unlike others in the genre, it took this trend to new highs (or lows).
It was so lampooned by critics that director Michael Bay officially apologized for it. It was so lampooned that one considers defending it out of pity. Well, that is, until you see the money it made. But, hey, you can't buy respect.
And how about an explanation of why it was so extraordinarily successful? "Star Wars" has its legacy, but this franchise has ... ?