Most difficult orchestral piece

For me personally, ALL orchestral work is considered the most difficult to perform

Some of the answers to this question on Yahoo answers might inform:

http://answers.yahoo.com/question/index?qid=20080702090955AA5H7jh

I think Schwantner would get my vote too based on the description of it.

It depends on one's criteria, of course. For sheer number of musicians to manage, Mahler and Bruckner symphonies have always been challenging for conductors. They can sometimes be endurance tests for musicians, too, though these works are such staples of the repertoire that experienced players come to be pretty familiar with them. Similarly, there was a time when Stravinsky's Petrushka and Rite of Spring were considered tours de force for musicians because of their rhythmic and metrical complexity, but these days (especially now that both pieces are almost a century old) conservatory students can play these works in their sleep.

Taking into account the quantity of musicians to be managed, the technical demands of the music, and the "freshness" of the work (meaning a work performed infrequently enough that it still comes as a surprise to most musicians), I would guess that Ives's symphonies, Messiaen's orchestral works, and Schnittke's concertos are pretty high up there.

For the virtuoso, nothing is difficult. For the hack, everything is difficult.