Who is the best vocaloid?

Best is relative and could cover a variety of qualities.. Best default vocal clarity? Best vocal range? Best high range? Best low range? Clearest pronunciation? Best overall in many categories? It really depends what you are looking for.

With the latest Vocaloid 3 voice libraries, and with the general improved quality that the Vocaloid 3 engine brings, even converted Vocaloid 2 libraries sound generally better with V3. However, you can't go wrong with any of the true Vocaloid 3 voice libraries with the deciding factor being voice characteristics.

V3 libraries released so far as of 7 Feb, 2012, they are:

IA - Japanese

Rion Tone - Japanese

Yukari Yuzuki - Japanese

CUL - Japanese

Oliver - English

Bruno - Spanish

Clara - Spanish

Megpoid GUMI Power - Japanese

Megpoid GUMI Whisper - Japanese

Megpoid GUMI Sweet - Japanese

Megpoid GUMI Adult - Japanese

VY1V3 - Japanese

Mew - Japanese

SeeU - Korean / Japanese

Several more scheduled for release in 2012: Lapis Aoki, Suzune Ring, Lui Hibiki, VY2V3, English Miku. V3 Lily, and the first Chinese Vocaloid. While Luka Append, Meiko Append, Kaito Append, are also slated for this year or early next year, it isn't known if they are actually going to be V3 voice libraries or still using V2.

the most popular vocaloid is Hatsune Miku. But she was not the first one. The first vocaloids were in English like: Sweet Ann, Kaito and Meiko. She is very popular, along with Kagamine Rin/Len. and vocaloid No. 3 is Megurine Luka. You can look all of them up on the official Crypton website in Japanese. My favorite one out of all the vocaloid is Rin/Len Kagmine. there are other vocaloids like:

Gumi Megapoid

Gakupo Kamui

Miki

Haku Yowane

and much more