As a child, it was hands down The Little Mermaid. I begged my parents to buy me sheets for my bed with LM designs. (I was quite young at the time).
I also loved Lady and the Tramp, as well as The Rescuers (not so much Down Under).
When I was a little older, my favorite was Mulan (still is quite high).
These days, well, I love the songs from Beauty and the Beast more than any other. The character development is more multi-dimensional there than almost any other story.
I did not like Hunchback at all, nor any of the Aladdin movies, I couldn't stand Hercules, nor The Emporer's new Groove. And I especially despised Lilo and Stitch.
Wreck-it-Ralph was surprisingly good, and one that I'm sure I'll watch again... But as a movie and as a story... It doesn't really hold up to some of the earlier Disney pieces.