Will -- NAOKI. I am a fan of what s/he has contributed to the Dance Dance Revolution series.
Just before that: Silly-Go-Round -- FictionJunction YUUKA. Not a fan of her, but I can't shade the song itself.
Being fairly untrustworthy of non-regular vocalists and having never seen Ordinal Scale, I've only just come across Longing and Delete (which I'm listening to now and have been for the last 20 minutes).
Before that I was listening to music from SSX3, a favorite game of mine, my favorite song from it being Poor Leno by Royksopp.
Point Zero, Live arrange version.
The first time I listened to this, a few hours ago, I was chopping vegetables, and cut myself because it was just so intense. Never again cooking while listening to the Fate/Zero OST.
Samples of Aika Yoshioka's songs on iTunes (They have a surprising amount of Japanese stuff, I am amazed) I wish so badly that Yuki would work with her. I think her voice would fit a Yuki song so much.
Double post because I love this song: Sonogyaku-Chiaki Ishikawa I am sure @Kugayama has heard it. I love it and just got it today! It's almost like it could be in a fighting anime or something. So good!
Aikoi LIVE -- FictionJunction YUUKA.
Although my tolerance of Yuuka is miles higher than my first discovering YK, I can't sit here and say I don't watch her live performances just to see and hear what Yuriko Kaida and Hikaru are doing in the background. That "hayi ayi eyes-to-me" is literally my favorite part of the song and I usually rewind it about 6 times.
Just before that:
The "FictionJunction The BEST Live #2" Pandora Hearts trilogy of Alone, Bloody Rabbit, and Contractor.