Mine is definitely Gumi, she has the best tone and voice over all. Also like Rin and luka. Too bad no one uses anyone but Miku who i find to be one of the worst vocally. Very rarely is she tuned well and she have little range outside of higher octaves. People always use her but do it terribly. i like Len, Kaito, and Gakupo.
I've been getting into uatu more so though.They typically sound more realistic than vocaloid. Plus its free and the community is all about sharing and making things for everyone to use and create with. Ritsu Namine is unbelievable sounding.
https://www.youtube.com/watch?v=E2jb5TbJk9ghttps://www.youtube.com/watch?v=auyg9102uz0Even others sound way better.
https://www.youtube.com/watch?v=QH21xtIRzmwhttps://www.youtube.com/watch?v=yXfXnGKFxZ4