Mulan.
Not to get all PC on people, but for me it was the first one where the Princess didn't have to faint and waste away waiting to be rescued by a man...I think every little girl should be made to watch it twice for every time she watched all the others (which I also love, btw)
On another note, it was the first movie I ever took my oldest child to see in a theatre. ;-)
Sorry, gotta add my list of movies my kids aren't allowed to watch...ever. #1, Pocohantas (Hey, you don't get to change history. Yes, I've told my sons that her tribe basically sold her and she died three months later) and #2, Hunchback, (you don't get to change a classic work of literature. And yes, I have told my sons that in the real story, everyone burns.