User blog comment:ThePK/THE RESULTS: Vocaloid Community Choice Awards/@comment-217.38.103.99-20150723182157/@comment-5051846-20150803123434

Although any Vocaloid can sound realistic with good tuning, Miku isn't generally treated as realistic because she isn't designed to be realistic. That's partly why Crypton chose a voice actor to voice her, rather than a professional singer. She isn't actually supposed to sound human, though she can achieve realism with a dedicated producer. ヽ(^o^ヽ)

If you want a realistic Vocaloid it is best to go for one designed for realism, as Miku's strong point is her voice's character; her unique android sound is basically her selling point. :D