I'd say this one, along with Bambi, Fantasia, The Lion King and The Little Mermaid :3

Nah, definitely not, it's one of my least favorite actually. Too dark, plain and life-less.
Pocahontas has the best animation for me.

It was a breakthrough back then, in comparison to Snow White it was incredible, but there's been many movies since then that have even better animation.

I think there are others that surpass it easily, but the area in which I think it's a breakthrough is how some of the characters are drawn in a slightly quirkier style than anything the company had produced up until then. Artists opting to create characters using straighter lines and pointier shapes. I especially like how they drew Samson and dragon!Maleficent. That kind of art style has only been used in a handful of Disney movies (other examples include 101 Dalmatians, Hercules, The Emperor's New Groove, and Home on the Range) but I think Sleeping Beauty was probably the one that first used it or at least defined it early on.

The animation is nice, but not my taste.