Diet versus exercise, what contributes more to pure weight loss?

Iirc, Scooby once said that you can pretty much lose all the weight you want based on diet alone.

@Khaini: I've never heard about blood types affecting nutritional requirements. I thought it was solely based on cutting down depending on your daily caloric intake and daily caloric requirement. I've only ever heard of blood type affecting transfusions, not weight loss...