This is going to be hard, because each of them hit some kind of milestone that made them truly important in Disney's history.

Aladdin or Princess and the Frog.

Disney was fine before it came out, not saying that PATF ruined it because I know my comment kind of sounds like it

Disney wasn't fine before The Princess and the Frog came out. It was fine before the 2000's hit.

^^oh, okay maybe I'm changing my pick lol

Not important

Or Pocahontas.

waaaaaah! goodbye, Aladdin. :(

Snow White was the first
Cinderella was the iconic
Sleeping Beauty was the most uniquely designed and executed
The Little Mermaid was a return to that original fairy tail style which helped Disney out
Beauty and the Beast was considered the best overall film and Disney's Magnum Opus, garnering an oscar nom even
Aladdin started the theme of having more well known actors in the films, which continues to today.
Pocahontas was the first non-european
Mulan was the first to have the heroin really being the heroine
The Princess and the Frog was the first African-American princess and was the first well made 2D film since Lilo and Stitch. (quite a while between the two)