In your opinion what is one of the greatest orchestral pieces of all time?
One that comes to mind that is one of the greatest to either hear in concert by a great symphony orchestra, is the "1812 Overature" Of course it is also one of the best if you are in the orchestra as well.

Dance of the Sugar Plum Fairy. Does that count? :confused:

Oh, where is Scarampella when you need her?! ;)

"O Fortuna," the opening piece of the suite "Carmina Burana" by Carl Orff

Beethoven's Third Symphony, "Eroica"

"Jupiter" from "The Planets," by Gustav Holst

George Gershwin's "Rhapsody in Blue"

Modest Mussorgsky's "Pictures at an Exhibition."

Oh there's a lot that I like.

Beethoven's Symphony #5 - most famous and most well-known of his masterpieces.
Beethoven's Symphony #6 "Pastoral"

Schubert's Symphony #9 "Unfinished"

Any of Richard Strauss' works, like Danube Blue.

Just to name a few.

All of "Carmina Burana"

Holst's "The Planets" (all movements)

Johann de Mejj's "Lord of the Rings" (again, all movements)

Anything written by David Maslanka (a superb contemporary composer)

Mozart's Requiem.

'O Fortuna,' is mine too. Very epic.

I was always partial to Ruslan and Ludmilla too.

Hell's Kitchen-Dream Theater.
Oh if you are talking about classic music then it's Vivaldi's The Four Seasons.

@Elios
Bah. The 1812 Overture is overrated... that or I'm just pissed that I cannot find a high quality recording with both chorus and artillery. :)

Anyway...
Rachmaninov's Symphony No. 2, more specifically the third movement.
Rachmaninov's 3rd Piano Concerto.
Beethoven's 9th Symphony, more specifically movements 2 and 4.
Te Deum by Verdi, from Quattro Pezzi Sacri.
Pretty much all of Messa da Requiem also by Verdi.

Just to name a few...

Elios, didn't you know, nobody listens to classical music!

Gustav Mahler Symphony #1,2,3,4,5,6,7,8,9
Richard Strauss Also Sprach Zarathustra
(theme from 2001: space oddessy )
All his Operas
Igor Stravinsky the Rite of Spring
Sergei Prokofiev Romeo and Juliet
Symphony #5
Johannes Brahms Symphonies #1-4
Ludwig Van Beethoven Symphonies #5, 9
Amadeus Mozart Requiem

just a few of my fav's