I don't know! There are so many good ones and I think it is constantly changing for me. The following are some of my favorit in lyrics, sound, and visual.
"Key" oleh Daisuke Ono (Psychic Detective Yakumo) "Nirvana" oleh Mucc (Inu X Boku SS) "World's End" oleh Flow (Code Geass) "Spell" oleh LAMA (NO.6) "Closer" oleh Inoue Joe (Naruto Shippuden) "Things I can do" oleh HOW MERRY MERRY (Natusme Yuujinchou) "Stray" oleh Steve Conte (Wolf's Rain) "Zero!!" oleh Minami Kuribayashi (The Devil is a Part-timer!)
But I'll go with "Izayoi Namida [Hesitating Moon]" oleh Yoshioka Aika (Hakuouki) because I could just listen to it over and over again. And I cinta the lyrics! It basically means time is fleeting and bunga don't last forever. But spring always comes and the cycle of life will continue again.