Song of the south. But sadly the public will never again see it because the NAACP considers it to be racist in nature. Few even know that the famous Disney song Zippidy Do Dah, came from this very movie!
Snow White. It was the first Disney movie ever made. Although Cinderella would be a classic too, since its become so widely popular and there are so many different versions of it. I believe that whichever ones that were made during the time Walt Disney was alive should be considered classics.
"Bambi" was the first movie I ever saw in a theater; I was very young, about 4 or 5... still remember it well... and yes, I cried. :( Also saw Snow White and Sleeping Beauty way back then, too. Watched those with my mom at our public library when I was a very little ABU... :)
Radon_Ryan is right about the ability of new movies to be what is known as "instant classics"... for instance, I LOVE Monsters, Inc. But for old classics, I'd say that Lady and the Tramp or 101 Dalmatians would have to be among my faves.
Snow White (the very first), Pinocchio, Cinderella, Peter Pan, 101 Dalmatians, The Aristocats, Sleeping Beauty, The Jungle Book, and many others, including live-action movies like Island at the Top of the World, Bedknobs and Broomsticks and Mary Poppins