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