Favourite "low" moments (Please read. Probably not what you think.)

[Heck148]

My all-time favorite low passage tho, has to be Berlioz "Symphonie Fantastique" mvt V - the Dies Irae tune sounded forth by 4 bassoons [lowest register], and 2 tubas, an octave higher...This makes a huge sound if done right - the tubas provide enormous resonance, while the low bassoons can put plenty of bite, "edge" on the sound...you have a very centered sound, plus huge resonance around it...wonderful effect.
forget the ophicleides, they sound like crap - Berlioz ditched them ASAP as soon as the valved tubas became available.

[Maestro267]

Another one, because why not:

Havergal Brian, Symphony No. 1, third movt., the trombones (including a contrabass), with some very low notes, both pianissimo (near the beginning, with harp octaves), and fortissimo (during the famous xylophone solo, building up to the martial climax).

[jochanaan]

Oh boy!  Lots of low moments!  Let's see:

Bartok: Piano Concerto #1, opening
Bartok: Sonata for Two Pianos and Percussion, opening
Debussy: La Mer, opening of first and third movements
Haydn: Symphony #3 "Drumroll", opening
Mahler: Symphony #1, opening of third movement
Mahler: Symphony #2 "Resurrection", any number of moments in the first and last movements, plus the very end of the third movement
Ravel: Mother Goose Suite, Conversations of Beauty and the Beast, contrabassoon solo
Shostakovich: Symphonies #2 and 10, openings; Symphony #4, opening of third movement, and especially #13 "Babi Yar", opening of fourth movement
Tchaikovsky: Piano Trio, closing; Symphony #6, opening and especially closing

Trivia note: In the aforementioned Tchaikovsky #5, the instruments playing are clarinets, violas, cellos (divided in 2), and basses.