Board Thread:General Discussion/@comment-3571637-20150627003035/@comment-26239285-20150706083631

If the poll on VLW is anything to go by, in decreasing order it's Len, Miku, Rin, Gumi, Luka, then the others.

Obviously people have their individual preferences but perhaps more objectively it seems to follow the versatility of the Vocaloid as well as their suitability for a wide range of music (although which of Rin and Miku is more versatile is arguable). Just a theory.