I love his Requiem - must be performed in a cathedral! The brass ensembles calling to each other during the the “Tuba mirum” section of the “Dies irae” sends chills up my spine.
One is the Lacrymosa from the Requiem. It's in 9/8, but it's not in the traditional 3 groups of 3. It's 9 straight beats with the strong beat on 6. Near the end, there is a titanic struggle over E (brass) versus F (chorus) as a heaven-versus-hell battle. Then for the crowning touch, Berlioz staggers the chorus and orchestra a half beat off each other down the A major scale to take advantage of the echo in the hall (or church). It's one of the most amazing moments in music.
The Dies Irae from the Requiem is another amazing piece. He has a separate section of timpani on stage along with 4 brass bands in the balconies. Choristers have told me that the din is so trmendous that they are totally dependent on the conductor to stay together. Before the brass bands come in, the sopranos work the theme (A minor), then the tenors take up the march (B-flat minor), and the chorus works it up as a canon (D minor). It abruptly switches to E-flat major ("Tuba mirum spargens") as the brass bands echo off each other in fanfares, climaxed by the chorus singing over the drums. There is a short transition in A-flat minor ("Mors stupebit") before we go through the brass band section again, but with the basses taking up a chant along with the brass. The piece ends quietly, as though the chorus and orchestra are too shocked by what they have just performed.
The Sanctus from the Requiem features a tenor suspended from a catwalk and the only complete fugue in the piece.
His song cycle, "Les Nuits d'ete" shows a saner side of the composer. Elly Ameling's performance is prety much definitive.