Board Thread:Poll Archive/@comment-30602727-20150514233129/@comment-5051846-20150515174820

@Nick: We can get a good estimate on who was the best seller by looking at hits on Nico Nico Douga. Of course, this isn't perfect, and can only be applied to Japanese Vocaloids. However:

426 videos: 巡音ルカV4X

235 videos: VY1V4

95 videos: 結月ゆかりV4 (I'm not sure about this one, as Yukari doesn't have a V4 product name like the rest, and has no dedicated V4 tag)

Gackpo has been released too recently to know yet.

Going by this, it looks like Luka is the most successful by a long way.