Depends on how you define "animated" really, in order to say what is truly the best. If we're speaking strictly from a technical aspect and how flawless and well drawn/animated a scene is, then yes I would agree that Once Upon A Dream is indeed impeccable along with Colors of the Wind (stunning to watch), the Firebird Suite in Fantasia 2000, the Evil Queen's transformation scene in Snow White, Maleficent's battle with Phillip following his escape, the underwater sequences in Pinocchio as well as the climax with Monstro, Night on Bald Mountain - mainly Walt's era as it is done entirely by hand much before computers, making it all the more impressive; true works of art.
However, if we're defining the greatest animated scene based on the ability to emotionally connect to characters in scenes when they aren't even living or breathing yet feel so much more than just moving drawings because they are given life and believability by the animators, then that to me is what truly defines the greatest animated scenes. Glen Keane is the master of this! He literally gives his character's souls and captures it all on paper, they always have such expressive, emotive eyes (Beast & Tarzan especially!) and he can give so much feeling in such little animation, it's truly remarkable and his gift, it's pure magic to watch. Greatest scenes for me are Pocahontas and John Smith meeting for the first time, Savages Part II and Pocahontas saving John Smith's life (with "execution - Colors of the Wind / If I Never Knew You" score) and farewell (the score building as she runs to the cliff side is amazing, always gives me goosebumps; the musical score paired with the animation of her engulfed by the wind through to the sails of John Smith's ship), Tarzan meeting another human for the first time and having a physical connection with Jane when they touch hands, Kala teaching Tarzan that he is no different and that they're all the same (2 hands, 1 heart) followed by You'll Be In My Heart lullaby, Ariel's grotto and yearning to be a part of another world, Beast's death and resurrection.
Other great scenes include The Lion King opening sequence, Dumbo Baby Mine, Marahute and Cody flying, The Hunchback of Notre Dame Hell Fire (very powerful animation along with fantastic music), Widow Tweed letting go of Todd - although not visually the best animated sequence of all time, it's still incredibly well done for a simple scene (the music and animation never fails to make me weep
), The Rescuers also has scenes like that too.
