Not many masterworks are written as a challenge, but that was the genesis of "Spem in Alium," composed for 40 voices (eight choirs of five — sort of early surround sound), and still one of the greatest achievements of English music. Clocking in over 12 minutes, it builds from shy ambience to orgasmic, melodic majesty before slowly falling back into the distance. Revolutionary when first performed, it's still music to take the breath away, graceful, elegiac, and as timeless as all great art. The rest of the music here is delicious, but "Spem in Alium" does that rarest thing — it touches the sublime.