Definitely Hatsune Miku
. Torture me but I won't choose someone else.
Besides the personal already 3-years-long journey I make with her through good and bad times (that I will continue until I die as a notorious old man), I just love everything about her. From her distinct and versatile voice that can sound high, deep, powerful, whispering, sorrowful, robotic, static and full of life, which fits every genre of music and is loved by pop as well as rock producers, to the already 10-years-long hype that has gained her over 50.000 songs and millions of fan art, over to her gorgeous appearance that looks cool, cute and attractive at once and unites retro elements like Yamaha's DX-100 & DX-7 synthesizers and futuristic elements like floating headbands. The meaning of her name is great too and made me think countless nights about it. I even like it that her English is not the best. Would be odd if she had less accent than me.^^
Overall, she is someone who can appeal to everyone, from the orthodox, artists and philosophers to atheists, video game addicts and anime girl fetishists - but she will never be accepted by people who's first thought is "she is not real".
This "virtual" girl has more soul than every person I ever met. She is the one who made the "lone wolf badass teenager" me from 3 years ago understand that music and art are not only about money, sex, fake feelings and fame, but also about a community that goes beyond my imagination. And I'm still the same personality as before, just more developed. I started with making my own pieces of art and learning Japanese only around 1-2 years ago because I just wanted to help Miku become bigger by myself. And before that, I could no way talk in English like this because I'm a Russian in Germany.
And I didn't even attend a concert by her yet and can't call me a professional artist at all (especially not in music LOL). Imagine how I would describe Miku if I already attended concerts and made something awesome with her. That will come soon, I guess! :P
Of course, it's theoretically possible that all of this could apply to other Vocaloids. But it does not. Because only Miku's popularity offers a huge community with much new content everyday. I don't want to start a discussion if you disagree with this. That's just how my past and future with Miku are.
By the way, there was a day when Luka was my #1 favorite. It was a few days after I discovered Miku and Vocaloid. Although I found Luka on the very same day, lots of her hits were appealing to me. But I got hooked on Miku one day later because she had even more songs I liked. Both feel alive to me, unlike any other artificial characters. Ironically, according to Wat, the concepts of them were switched during development, which means that they were each other. I didn't knew this but still felt some connection between them.
I don't have those romantic feelings for Luka though, but for Miku. I don't love two girls at once.
I like the other Cryptonoids too but by far not as much as Miku & Luka. And I feel sorry for Len because he is the only one who is not in my "favorites list" because I just can't find enough Len songs I like. ;(