I think most people are conflating theme (music) with theme song. They're different for these awards.
And like it or hate it, Celine Dion's "My Heart Will Go" is got to be biggest commercially, "Beauty and the Beast" is great, "Moon River," "When You Wish Upon a Star," "As Time Goes By," and "I've Had the Time of My Life" are iconic, but . . .
the greatest song in movie history is Judy Garland's "Somewhere Over the Rainbow."