I do so hate these polls, but if I were to be asked at gunpoint to select one piece in this one, it would have to be Sonata No. 8. Like No. 10, the hypertense febrile helter-skelter of fragments that occurs towards its end is monstrously hard to control, let alone render meaningful and the oft-repeated (in different guises) passage in fourths is also notoriously tricky to bring off every time without fail - and these are just two examples of what makes that piece a real challenge, even by Skryabin's exalted standards.
Best,
Alistair