A lot of idol group changes their center depending on the particular song and performance they're looking for, so Ruby will have her time to shine later. For now Kana would be the best choice to go as the center of their first-ever performance.
@esega It makes sense really, she hasn't really done anything yet to prove her worth. You can't really expect someone to see something in a person who hasn't had the chance to show off yet.
@Dastron same, I remembered that it was Friday and saw this chapter was released after I finished reading something else. Mangadex has had problems showing that new chapters have been out for stuff
Best Dancer or Best Singer rarely does seem that important for jp Idol groups at Center.
More important I'd say is potential and Aura, but of course that's hard to define. xD