I’d have to say either Gumi, because of her songs, or Miku, for having led the Vocaloids so far, in terms of popularity. I know a lot of people dislike and some even downright hate Miku because they feel she’s hogging all the attention and stuff, but I feel that she’s sort of the mascot, the representation. It’s her that’s got them all this far. I mean…Meiko and Kaito came out before Miku but it took Miku’s design to get the characters actually popular. But that’s just how I see it.

So, long story short, it’d have to be either Gumi or Miku. Thanks for the question, @Seekernest !