Hardness, in my opinion, is difficulty per unit time integrated over event time, in which case I think the marathon would be hardest.
As for which event reaches peak difficulty per unit time (for me personally anyway), I would have to go with the 800m (could be 1500m depending on the day). Shorter than that doesn't allow the effect to build up all the way, and longer than that requires less effort per time in order to finish with the fastest time possible.