Kaito

Or Kaito Shion. Shion is a fan-given last name meaning "first sound", since he was the first male vocaloid. Originally he was meant to be MEIKO's counterpart, but he's my favourite. He sounds good with himself (ah, technology ), Len, sometimes Meiko. Links to some of my favourite songs by him (most songs by any vocaloid you can find by any of the other main ones):

Tell Your World:
Akatsuki Arrival (two KAITOs):
Systematic Love:

Snow White Princess:

And Snowman (duet with Len):
[60fps Kaito Len] Snowman スノーマン - カイト 鏡音レン Project DIVA Arcade English lyrics Romaji subtitles PDA

Promise with MEIKO:
【Project DIVA F 2nd】 Promise 【MEIKO・KAITO Cover】

1925:
[60fps Full-cast] 1925 - 初音ミク リンレン ルカ KAITO MEIKO Miku Rin Len Luka DIVA English Romaji subtitles

Sing & Smile (full cast):
[60fps Full-cast] SING & SMILE - Project DIVA Arcade future tone English lyrics Romaji subtitles

Erase or Zero with Len:
[60fps Full風] erase or zero - カイト 鏡音レン KAITO Kagamine Len Project DIVA English Romaji PDA FT

and monochrome dream eater:
[Project Diva Arcade] Future Tone "Monochrome Dream Eater" Kaito

Just for fun Snowman full cast:
AND THREE KAITOS??? :
Knife ,Kaito V1 ,V3 Soft et straight cover HD (sub English )

The first song I heard him in:
[60fps Full] カンタレラ Cantarella - Hatsune Miku KAITO 初音ミク カイト Project DIVA English lyrics Romaji PDA

And this one when I'm in the mood for it:
【Project DIVA Arcade】Pane dhiria【PV】KAITO - スミレ中文字幕

Or this:
Project DIVA Arcade Future Tone Sweet Devil Kaito Cover

And his sass battle with MEIKO:
[ KAITO x MEIKO ] World's End Dancehall PDf