To explain why I picked it about Snow White: It was groundbreaking, too, but I think that somebody would have made an animated movie sooner or later. In fact, there allready were animated movies. But BatB was the movie which earned the animated movies respect, which is quite a feat. Plus, it changed the perception of what was possible in animated movies concerning camera angles.

The very first animated movie to be nominated for Best Picture at the Oscars... need I say more

For obvious reasons.

The Little Mermaid started the Disney Renaissance and brought Disney out of the dark ages. Ariel IMO is an amazing character who broke free from the traditional princesses. She is a proactive character and makes her dreams a reality instead of waiting for them to come true. This was the movie that returned Disney to the musical format it was famous for. Maybe without the Little Mermaid, other amazing movies like Beauty and the Beast or Aladdin might not exist. I'm not saying they would never exist, but maybe things would be different without The Little Mermaid.

I wouldn't call Ariel proactive...most of her decisions are influenced by other persons.

The argument again "The Little Mermaid" is already in the word "Renaissance"...it was a revival of something which allready existed beforehand. The princess movie, the musical format, nothing about it is new, TLM just modernized it a little bit. That's hartly groundbreaking, especially since the claim that this movie started the Renaissance is disputed.

Ariel is proactive, she goes after her dream instead of waiting for it to come to her.
Your right, the musical format wasn't new, it was used throughout the 1930s-1970s and the less successful Oliver and Company the year before. With The Little Mermaid Disney started once again creating movies based off fairy tales and well known stories and brought back public interest in Disney. I think that's pretty impressive. But, i'm not trying to say Beauty and the Beast wasn't groundbreaking. It was.

I agree with Swanpride and Mongoose09.

or Snow White IMO :)
I also agree with anonheart998

anonheart998 couldn't agree more although if it was possible to pick more than one I would vote three movies: TLM, BatB and Tangled because each of them was a significant milestone not only for Disney company but for animation in general.

No "persons" influenced Ariel's long term dream of being a human as well as her determination to collect as much information about the human world as possible - all of it prior to even laying her eyes on Eric...