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