Personally, I thought the first two Fox movies captured the FF pretty well; but it was the FF of 1963. When FF first started in comics, it was not serious at all. In an early story, Doctor Doom was defeated and left on an asteroid to die in deep space. A few issues later, we open with a scene of Stan Lee working in his office when suddenly Doom bursts through the door. Stan is shocked and exclaims "How did you escape that asteroid?!"; and his answer is "Doom explains himself to no man!" before Doom grabs the phone and crank calls Reed. Seriously. That was it; and that's what Fantastic Four was.
Despite it's world endangering plots, Fantastic Four was conceived to be light. In truth, we have already had a very successful Fantastic Four movie - it was called The Incredibles. Brad Bird embraced the concept for what it was meant to be and people loved it. Of course, Bird also had the opportunity to put the Incredibles in a world populated with other heroes; and that is an element to Fantastic Four. The FF has often served as a bridge book with all of Marvel's characters filtering in and out.
