NOTICE: This question falls into the FAQ page...
What type of questions can't be answered by Vocaloid Wiki
Questions on Personal (issues / opinions / feedback) about Vocaloid series/products
Note Please view the individual Vocaloid pages on the Vocaloid Wiki, the section for popularity gives some insight. And unless you are specific the term popularity can be many things, such as "Popular by...":
- Origin (company)
- Listener stats/views
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 that while Hatsune Miku is the most known, it is the Kagamines that are also highly favored.