2.Main Discussion:

Imagine you could only master one skill completely, while the others remain at your current level. Which one would you choose?

Explain your reasons clearly. Try to persuade others who might think differently.

I would choose listening, because I think it is the most difficult skill of language learning to improve

Understand your point, but I wonder what if you face too many vocabularies you don't know? One cannot understand a listening text before it knows important words in it.