My favorites would be:
"Aiai Kasa" (Neo Angelique Abyss) - Tegomass
"Aozora no Namida" (Blood+) - Hitomi Takahashi
"Colors of the Heart" (Blood+) - UVERworld
"4th Avenue Cafe" (Rurouni Kenshin) - L'arc~en~Ciel
"Kimi ni Fureru Dake de" (Rurouni Kenshin) - Curio
"Half" (Rurouni Kenshin) - Kawamoto Makoto
...and for my all-time favorite....
"Anbaransu na Kissu wo Shite" (Yu Yu Hakusho)
by Hiro Takahashi
^.^