I'll take this as one that sticks out in my mind.
For the new millennium, I like '01 Cadets. Every element of that show is taken to the musical limit as far as difficulty. Young Person's Guide had the complex rhythm, Vide Cor Meum requires such control to keep the emotion pushing like they pulled it off, and Farandole at the tempo they took it was insanity. Moondance was fun, not nearly as technical as the others, but the whole package was fantastic in terms of difficulty.