I was actually a bit misleading in my post below about the top movie flameouts, because my original source didn't take recent releases into account. When you do, as Haiwen pointed out, you find that the top flameout of all time isn't some bloated special effects blockbuster, but a critically acclaimed and well-respected popular success: 8 Mile, which earned a whopping 44.2% of its final $115,000,000 gross during its opening weekend. That's pretty amazing for a movie that most people actually seemed to enjoy.

As for the other top flameouts, they are, in order: Godzilla, The Lost World, Planet of the Apes, Batman & Robin, and Pearl Harbor, about which no comment is necessary, I think.