Difficult choice.
I think Cinderella would have a lot of potential to be a very "magical" dark ride. Lots of possible transformation effects (Pepper's Ghost?), wonderful music, some of the most memorable scenes of any Disney movie. Not to mention a very strong existing connection to the Magic Kingdom. In terms of feasibility this one may be a winner; I feel like Disney could pull this ride off with the technology found in the older Fantasyland rides.
Sleeping Beauty could be one of the most visually beautiful dark rides ever made. To ride through a three-dimensional realization of
Eyvind Earle's artwork could potentially be one of the most stunning visual experiences ever placed along a ride path. Some great music too, including the often-overlooked
Sleeping Beauty, Sleep On. Not to mention the possibilities for a huge AA dragon and pyro effects.
Beauty & the Beast has potential to have huge audience appeal given its more recent release. Also, so many of the characters are household items, there's great potential for a highly realistic dark ride with AAs that look dead-on like characters from the movie (whereas Sleeping Beauty would have to be more stylized). A projection-mapping castle transformation could be very cool. This movie would probably need something closer to Mermaid in terms of scale; I'm not sure Peter Pan-style could quite pull this one off.
Surprised at all the votes for Alice in Wonderland. Considering we'd have the choice for ANYTHING, I'd much prefer to see Disney try a dark ride with all new technology and based on a movie they've never used before, vs an existing ride from 1958 you can see right now.
EDIT: Also surprised to see that all three of my choices have the lowest votes of any of the poll options. Oh well, them's the breaks.