In terms of sales, Hatsune Miku is the number 1 selling Vocaloid (60,000+ units). However, no other sales figures have ever been released on Vocaloid except Kaito (500 units) and Meiko (3,000).
We do know that Vocaloids need to ship 1,000 units to be declared commerical successes and that Leon, Lola and Miriam were not commerical failures. Also, Prima and Sonika were in Zero-G's top 25 products of 2011 list, meaning commerically they were successful last year.
These however are only fan estimates on the success of other Vocaloids.