Most Technically or Intellectually Difficult Film To Make

[David]

Cortot said that the two most difficult pieces to play in the entire piano repertoire were Balakirev's "Islamey" & Ravel's "Gaspard de la nuit."

Aside from such considerations as funding & wrangling with temperamental studio heads or actors, what movie do you think was the most technically difficult to film? Would it necessarily be an epic, with the marshalling of forces on a grand scale needed? or perhaps something on a much smaller scale that nonetheless required such intensive, delicate mise en scene & shadings & planning that it was akin to working out an advanced astrophysical equation?