Time to get this thread moving again.
Have thought of several challenges for you astute pundits. The first is the answer to the question "what is the most difficult orchestral work?"
The rules require that it be played more or less regularly and that it be instrumental only (no solo concertos either -- we'll make that another category). Sorry, Mahler 8th lovers...
So I'll start off the sweepstakes with two examples: Bartok's Concerto for Orchestra and Strauss's An Alpine Symphony. The first has intricate contrapuntal textures and associated rhythmic complexity -- and each orchestral section is treated like a concerto instrument, thus the name.
The Strauss has tremendous bombast along with its technical demands as well as Herculean length and orchestral size. Neither of these opuses is comfortable for the average orchestra.
But you folks can come up with better examples, so I leave it to you to tell me the most difficult orchestral composition. And if you have an orchestra and conductor that tamed the beast, so much the better.