I've gotta go with "Live and Let Die". Everyone aknowledges it's a great song, but in my case I'm also a Beatles fan, and as a Bond fan, that movie was my introduction. As a result, I admit I may be biased by sentiment. "Nobody Does It Better" and "For Your Eyes Only" rank high as well.
On the other end of it ... that Duran Duran thing sounds to me like the band had two unfinished songs that they just mashed together ... and they were unfinished 'cause neither of them were any good. As for "Die Another Day", I assume that was to help us relate to what it's like to be help captive in a North Korean prison camp for 14 months.