I know it's kind of complex to answer such a question, because unlike today's features, older Disney films opened in a calmer way, or sometimes with a book opening, but still try to answer nontheless
In my opinion, "Beauty and the Beast" has the best prologue. The opening shot of the castle between the trees is strikingly beautiful and the whole stained-glass idea is brilliant. Gives a really artistic feel to the film.
Another one that starts beautifully is "The Little Mermaid". I love how they drew the seagulls flying in the sky and then Eric's ship bursts out of the fog with the sailors singing.
"The Lion King" is also a good example of a superb opening. The african views are realistic and breathtaking, easily drawing the viewer's attention. Classic.
I think that although a great movie, "Aladdin" starts very weak. Yes, it approaches the story in a very comic way but I find the whole scene of the narrator riding on a camel in the desert quite simply unimpressive. But of-course, it's still an amazing piece of work.

