It is really among Fukase or Una. I said Una because her popularity seemed to skyrocket out of nowhere.
I’ve heard a lot of mixed things about Fukase, and to me, he seems more popular in the West (likely for his English).
Una though has had a lot more marketing to boost her. From the song and illustration contests, to the commercials that came out of that, to her online store with merch. Not to mention having a Talk letting her get into Voiceroid communities too.
I also find that the most popular Vocaloids get into NicoNico Cho, and if I remember right, Una was in last year’s and this year’s? And I don’t believe Fukase has been, but he is also potentially held back by his special license.
Dex and Daina are obvious for Engloids, having the most promotion and allowing them to be boosted to popularity. For other languages, it’s not really easy to tell. I don’t think the Western community is close enough to the Chinese or Korean community to know how they’ve done.
Just from what I know, I’d guess Stardust did best but that’s based off the general of females doing better than males and two of the new Chinese Vocaloids are way too new to tell. VSinger as a whole is probably more popular tthwn Stardust though, but she still gets a good anount of promotion with figures, her official YouTube channel, and her luve concert. That’s really all I can say about it though.