The one that set the standard and broke the mold.  At 2:36, the longest score in the world up to that time, it was Max Steiner’s – the greatest film composer of the first half of the century (uh, yeah, the guy also wrote the music for Casablanca, King Kong, A Summer Place, and on and on) -magnum opus.  While the film is sort of caught in the dated mores of a long-gone world, the music – Steiner was the first composer to insist that each character have their own theme, a standard used even today -  and Clark Gable stand above it, separately, both timeless.