Ahhh, without a shadow of a doubt, Lilium from Elfen Lied.
I like almost all of the versions of it. Here's my favourite 3 versions:
Original -
YouTube - Opening Theme: Elfen Lied " Lilium " (Full Version)
Saints -
YouTube - Elfen lied- Opening Theme Song: Lilium (Saint Version)
Music Box -
YouTube - Elfen Lied - Lilium (Music Box)