That's a tough one, but maybe World is Mine. Or possibly Trick and Treat. Black Rock Shooter, maybe, but I also like all of the songs in the 'Evil' series. Dang, this is a toughie. I'd have to go with the 'Evil' series because those songs never get old, in my opinion.
I still like it, I just don't listen to it as much. It's still on my iPod. ;) I can see how that would get confusing... and I believe in Japan, Len's name starts with an R like Rin's. So that would just make it harder
Alice human sacrifice
But for every individual character...
Miku- Love is War
Rin- Kokoro
Len- Servant of Evil
Luka- Palette
Gumi- Lie and Diamond
Gakupo- Dancing Samurai
Kaito- Lord of Darkness
Meiko- My true self
either unhappy refrain by Hatsune Miku because it relates to my self or Worlds end dance hall by Hatsune Miku and Megurine Luka because i love the rythm and it has a powerful meaning or Never let you go remix by SeeU because it has a wonderful rythm and sounds soo emotional
Either Puzzle by Hatsune Miku or World's End Dancehall by Miku(I know you would say Luka's in it too but believe me. It's on iTunes and it said World's End Dancehall(Feat. Hatsune Miku)
Weird right?
Or maybe the song I don't care who, someone go out with me by Kagamine Len. That song made me crack up.