It will depend on the petticoat used. If you already have a petti you're trying to cover, it makes it a little easier to decide. Otherwise, it's entirely how you want your R2 to look. I think a bell shape would be the cutest profile since R2's rounded. One of the gathered skirt options would be easy, but a half-circle would be best. My experience: Lolita fashion and a whole lot of pettis and skirt shapes in my wardrobe.http://z3.invisionfree.com/Lolita_UK/ar/t35.htm
This link shows three common shapes.
petti is the most common and is similar or identical to the most commonly found pettis, like square-dancing ones. Malco Modes is a great company for those. A circle skirt will definitely work well with these and a half-circle will, too.
petti looks like the top of a cupcake and works well with a gathered waist rectangle skirt or gathered trapezoid skirt. These are a little more difficult to find outside of niche websites.
petti can be found easier than a cupcake and some A-lines can deflate into a bell shape after a lot of use, or lower-quality A-lines just naturally deflate to this. These work only ok for a circle skirt but look best with a half-circle skirt, a gathered waist rectangle skirt, or a trapezoid gathered skirt.