Debussy - Prelude a l'apres midi d'un faune
Tchaikovsky - the scene from Swan Lake (a duet for violin and cello) when Prince Siegfried and Odette dance together
Bach - 2nd movement of 3rd sonata for cello and piano
Chopin - Nocturne in C# minor
Tchaikovsky - 1st mov't of 6th symphony
Tchaikovsky - 1st mov't of Violin concerto
Bruch - 2nd mov't of violin concerto no. 1
Will probably think of more later...
oh..can't believe i nearly forgot:
Britten - War Requiem during the...
Ending - From the "Let us Sleep now..." in Libera me when you have the main choir going up and down scales, with soprano, bass (or baritone?), tenor, boys choir, full orchestra, chamber orchestra and organ) it's soo powerful (i sang it in choir last year and it's the best concert i've every been involved in, just standing in amongst all those singers/instrumentalists - there were literally about 350 of us, amazing - i actually couldn't sing at one point cos i thought i was going to cry).
Plus the choral acapella ending of the Requiem Aeternam and Libera me are so moving, the textures/harmonies are wonderful - give it a listen!
Love the Agnus Dei too although it makes me cry

I think what makes the War Requiem all the more sad is its subject. If you read the poems by Wilfred Owen alone they are sad enough,when you hear the poems set to music they become all the more poignant. Britten had the idea of bringing in a boys choir to represent the young boys who gave their lives to WW1. Their choral parts are often eerie sounding (enhanced by their high pitch), as though they are singing from the dead, like restless spirits. I think Britten originally wanted the boys choir to be off stage from the other ensembles to give a feeling of distance.