أنشئ حسابًا أو سجّل الدخول للانضمام إلى مجتمعك المهني.
I think the skill is listening
I think in all the four skills(listening, speaking, reading and writing). writing is the most difficult one because while speaking one can use the body language and tone to transfer the message. same with listening skill but when it comes to writing it is really difficult. Grammar, vocabulary, punctuation, clear ideas are needed to express and to explain.
In English I would suggest Grammar is the most difficult skill to master.