Anything with decent straps or a regular neckline. Sleeveless with a round neckline would be great.
Forget strapless or spaghetti straps. Those kinds of dresses only look good on the small-busted. A girl with a well endowed front end needs a good supportive bra, and that comes with straps. The dress needs to have plenty of fabric to cover the straps.
Another thing to consider is that your ladies probably have different "favorite" bras, thus, you can't just get one kind of dress that can be worn only with one kind of bra. Get something that will work with any kind of bra.
Dolly Parton is famous for being well-endowed. However, there are some movies where her character is not necessarily large busted. In these movies, the costume designers give her clothing that de-emphasizes her bust.
I suggest you go to a search engine and look for photos of Dolly in various movie roles, then take a close look at what clothing looks best on her.