What is your favorite Vocaloid song?

[Nana Shinu Ai]

Melt down miku
Cinderella and romeo with Rin and Len
World is mine Len  

[Emberxylic Len]

Kokoro - Len & Rin Kagamine
The disappearance of hatsune miku - Miku
Romeo & Cinderella - Rin & Len
Gekokujou - Len & Rin
Imitation black - Gakupo, Kaito, Len
3nodding 4laugh blaugh  

[Acheta]

Black rock shooter - miku
Alice of human sacrifice - Rin, Len, Kaito, Meiko, Miku
Daughter of evil - Rin
Servant of evil - Len
Re-birthday - Len
Trick and treat - Rin, Len
Alluring secret black vow - Rin, Len
Kokoro Kiseki - Len
Don't mylist me - Rin
Faint wish - Rin, Len
Madame merry go round - Miku
Imitator - Len
The seventh me - Len
I'll quit singing - Lily
Hymn of a gadget - Rin
Pretty panty*akuma rin - Rin, Rin, Len
Imitation black - Kaito, Len, Gakupo
Judgement of corruption - Kaito
Wood doll's song of martyr - Len
Prisoner - Len
Magical Neko len len - Len
The melancholy of detective Yowane Haku - Haku
Stop nagging me - Neru
Okay green is the enemy - Rin, Neru
Paradichlorobenzene - Len
Antichlorobenzene - Rin
Bye bye my blue bird - Rin
Kaito ga uninstall - Kaito

..... and some more which i forgot at the moment. And I like lots and lots other songs ^^  

[GAKUP0 KAMUI]

Dancing samurai?  

[Auspicious Beauty]

Delusion Sketch-Hatsune Miku
Nameless song-Kagamine Len
Vestige-Lily
Blue-Hatsune Miku
Rain in the country-Tourai,Jack  

[monkey17726]

So many...
Rolling Girl- Miku
KokoroxKiseki- Rin, Len
Trick and Treat- Rin, Len
Paper Plane & Prisoner- Rin, Len
Servant of Evil- Len
Re_Birthday- Len
I Like You, I Love You- Rin
Magnet- Rin, Len
Dreaming Leaf- Rin
Adolescence- Rin, Len
Gekokujo- Rin, Len
ARiA- Miku
Floating Love, Moon Flower- Luka, Len
Cinderella~another story~- Rin, Len
Melt- Miku
Meltdown- Rin
Love is War- Miku
Last Might, Good Night- Miku
Falling Falling Snow- Len
Imitation Black- Kaito, Len, Gakupo
Juvenile- Rin, Len
Gemini- Rin, Len
I stopped here, 'cuz my list would've gone forever--and I felt bad for having so many favorites. As you can tell, I listen to a whole lot of Rin, Len songs biggrin  

[Olapen Jove]

Meltdown/Kokoro - Rin Kagamine
SPICE - Len Kagamine
Judgement of Corruption/Another World is Mine - Kaito heart
Dancing Samurai - Gakupo
This is our Justice - Meiko
Insanity - Miki and Kaito duet
Last Revovler - Gumi  

[[ Final Omega ]]

There is no possible way for me to limit this down. I'd have to pick one for each Vocaloid, and even then it would be excruciatingly difficult to do. There's so many good songs and great artists! I'll have to get back to everyone later with a more concise answer!

At the moment though, I'm caught up on the following songs...
"Ur-Style" - Kagamine Rin
"Mozaik Role" - DECO*27 feat. Gumi
"Filozofio" - Megurine Luka
"I=Fantasy -breathing effect-" - SeeU  

[Serene Rainstorm]

Secret Police- Miku
World is Mine- Miku
Dream Meltic Halloween
Soundless Voice - Len
Proof of Life - Rin
Shinkai Shoujo - Miku
Time Limit - Miku
Anger - Miku
Saihate Ballad - Miku
Melt- Miku
Love is War - Miku
Fallen Down - Miku
Colorless World - Miku
Stargazer - Miku
After the End - Miku
Electric Angel - Miku
Winter Sky - Miku
Goddes of Spring to Moonlight in the Woods - Miku
Rainbow Butterfly - Miku
On the Outskirts of a Lamp lit Town on a Quiet Night - Miku
Sakura no Ame - Miku
Rolling Girl -Miku
A Faint Wish - Len
New Millenium - Len
Black Cats of The Eve - Len & Rin
Tautology - Miku
Chain Girl -Miku
Waiting in Earth - Kaito
A Silent Moonlight Night - Miku
Alone - Miku
Counterclockwise - Miku
Packaged - Miku
Melody - Miku
Moon - Miku
Fly With Me - Miku
After The End - Miku
Electro World - Miku
Alice - Miku
Innocence - Miku  

[Skeleson]

Gekokujou- Rin/Len and Meiko/Kaito v.
Melt- Kaito/Meiko
Hatarake NEET- Neru and Haku
Himitsu Kiestsu- Miku
Evil Food Eater Conchita- Meiko
Tsugai Kogorashi- Meiko/Kaito
Moziac Role- Meiko/Kaito
Witch Hunt-Luka, Gakupo, Miku, Len, Rin
Russian Roulette- Luka
Paranoid Doll- Gakupo
Ten Faced- Gumi
Matryoshka- Gumi and Miku
Bad Apple- Kaito/ Meiko
Take it Easy!- Miku
Bad Apple- Meiko/Kaito
World is Mine- Miku
Mosiac Roll- Kaito/ Meiko
Cantarella- Kaito/Miku and Kaito/Meiko v.
Kokoro- Rin
And many, many more :3  

[Nana Shinu Ai]

Melt down Rin
Rin and Len Kokoro  

[Nana Shinu Ai]

Kaito The world is mine  

[MagicAnimeGirl13]

Hatsune Miku's Bacterial Contamination!!!!!! emotion_bigheart emotion_bigheart emotion_bigheart  

[Hey_Wanek]

I love Eyesight test by Gumi. So cute! and and and I love Just Be Friends biggrin whee heart heart heart

I can sing both of those songs omg xd  

[Odd Cinderella]

English, Japanese, and Spanish versions of "Imitation Black."
Japanese and Spanish versions of "Love is War"
"Yellow" by: Hatsune Miku
VY-2 and Hatsune Miku's version of "Insanity"

And a couple more I can't think of... sweatdrop