There is no hardest, just different. what one person finds intuitive, others are left baffled and vice versa.
And what is "hard"? is hard trying to understand the turbulent flow coming off a fan blade by designing numerical simulations? is hard designing an elevator which has to carry several thousand passengers every day, up and down a 30 storey building? or is hard making sure that your customers get to watch Desperate Housewives via their Sky box in bad weather conditions?