My first and ever beloved Vocaloid: Hatsune Miku.
Although her songs are quite varied, I almost always like all of them.
From Tsundere, to twisted lolita. She's everything you want.
Some people might find her voice a tad bit 'unrealistic'. But honestly, looking at how other people make her sing, and how they make stories of the songs. She has a lot more substance than the other singers. She does not get paid for it, nor will she ever get into a scandal of sorts.
She won't age, the vocal qualities will depend on the user. (And more likely it's sub par or really above standards.
To simply sum it: I LOVE Hatsune Miku.