My two guesses: Beauty & The Beast or Pinocchio. The question only applies to classic Disney films, no Pixar. Besides the obvious factor of timing against past releases, these two hold a special place in the Disney canon.

Pinocchio

Walt was said to have described Pinocchio, his second animated feature after Snow White, as the story closest to his heart, and his quest for perfection nearly broke the studio. Pinocchio lost money in it’s initial 1940 release. However it did ultimately succeed, and Pinocchio is widely regarded as Disney’s most technically perfect animated film. Pinocchio was the first Disney film released on DVD, last available in 1999.

Beauty and the Beast

Beauty and the Beast was the film that saved Disney. Little Mermaid in 1989 was the prelude, showing that the studio was once again capable of producing an animated feature that could cross demographics, introduce lasting characters and revel unabashed in it’s own technical mastery. In 1991, Beauty & The Beast proved Ariel wasn’t a fluke.*

Beauty & The Beast was a huge hit, critically acclaimed and in every way earned its place in the Disney pantheon. Though he obviously never saw it, the story was dear to Walt Disney who twice attempted features based on the story. This was last available on DVD in 2002, and demand is still strong. We’re also approaching the time where the prime audience for the movie’s original release, 7-12 year old girls, are reaching the age where many will be starting families.

Currently I’m feeling about 60/40 Beauty and the Beast over Pinocchio. (I reversed that after writing this, will instinct or reason prove out?)

* I almost deleted that sentence.

Update, October 2008: And now we know. Disney’s first classic to be re-released in HD is Sleeping Beauty. Totally unexpected.