I feel like 灰姑娘 definitely. When people say the want a princess wedding, 或者 princess this, they almost always automatically think Cindy! She's not my most favorite, 或者 the prettiest in my opinion, but definitely the most timeless.
Beauty and the Beast. No contest. It's the tale as old as time, and I don't think anyone will ever get tired of it. Not to mention that Disney's gonna keep re-releasing anyway it because they're proud of the fact that it's the only classically animated film ever to be nominated for the Oscar "Best Picture."
I think that both 灰姑娘 and Beauty and the Beast are the most timeless. I can't really decide between them, they both have so many strong points and few weak points.