Among the four language skills which one gives second language learners the hardest time in mastery?

I think the skill is listening

Speaking I think which demands high confidence with all other expertise.

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.

Well. I think the skill is listening, because if you can understand correctly what people are saying, but you make mistakes while speaking (grammar, pronunciation, etc.) the other person can obviate your mistakes, interpret the message and understand you easily, and the conversation will flow with no major difficult.