The perceived value of a piece of music as "the greatest" depends on my current state of mind. It changes. Right now, I think Berlioz's Requiem is right there at the top, even though I don't care for his other compositions. Some days, fairly many in fact, I could be satisfied with Bach's solo cello works. Much as I love almost everything Mozart, his requiem feels too angry to me, so I hardly ever listen to it.