"Agnus Dei - from Requiem in D minor, Op. 48" (Gabriel Fauré)

It's a pretty safe bet that any work of music about death isn't going to be a light and breezy affair.  Masses for the dead, or Requiem Masses, often are centered, lyrically, around themes of suffering and harsh divine judgment.  That's because, traditionally, funeral masses all were based on Latin liturgical texts, particularly the "Dies Irae" ("Day of Wrath"), which is just chock full of images of suffering, destruction, and retribution for the sins of mankind.  (Even perhaps the most famous Requiem Mass of all—Mozart's Requiem—is laden with images of misery.  Take the "Sequentia" movement from Mozart's work, which contains elements of the "Dies Irae;" it is 8 minutes of full-on apocalyptic imagery.)  
Not exactly what a grieving loved one wants to hear in the throes of despair, in my opinion.
That's why I've always been fond of French composer Gabriel Fauré's Requiem in D minor (1887-1890, rev. 1893).  Fauré omits most of the hellfire and brimstone in his lyrics.  He focuses instead on Biblical passages that convey messages of love, eternal rest, and light, set to a score that shares the Romantic notes of late 19th century contemporaries like Debussy, Tchaikovsky, Satie, and Puccini.
One of my favorite movements from Fauré's work is the "Agnus Dei" ("Lamb of God").  I've had the chance to perform it a number of times over the years, so I know the music pretty well.  Nevertheless, its dramatic dynamic and melodic shifts never cease to surprise and inspire me, time after time.  
My absolute favorite part of the song comes around the 2-minute mark, where the tenors are singing the Agnus Dei theme, and the sopranos come out of nowhere, singing a single word on a sustained note: lux (light); it's the compositional equivalent of the exact moment at dawn when the sun peeks over the horizon.  As the note crescendos, the key completely shifts, and all of the other voices join in to bask in lux aeterna (eternal light).  
It may be the single most beautiful, most moving moment in all of classical music, which is saying a lot.