When it comes to companies viewing which of their Vocaloid products sale compared to others, that counts as popular to them. However, in an attempt to answer this question based on fandom reactions, it appears Hatsune Miku is the most known, with Kagamine Rin as highly favored with Kagamine Len afterward.
This is a listing of eight based on the fandom's familiarity with the following characters. An increase to make the listing a "Top 10" results in the ranking shifting between every Vocaloid currently released. This list can be viewed as outdated at any time due a Vocaloid's usage, songs, artwork, and fandom support for a month or year.
8.(Gackpoid) Kamui Gakupo
10.Lily and SF-A2 Miki