In your opinion which is the hardest task to learn either verbal or written English?

One can sharpen his/her written English in so many ways, including: Books, Computerized based trainings, penpaling...etc.
The case is totally different when it comes to verbal English which can be sharpened through only one way; that's "Practice with native English speakers".
Movies & songs provide a reasonable resource for practice, but to really achieve a real advance, face to face contact must be conducted.
That's why working on verbal English is much more difficult.

verbal,,, Because if you know how to communicate in English; writing issues will be only the spelling problems! But the opposite doesn't work, if you know how to write doesn't necessarily mean that you know how to talk, because of the pronunciation which happens to be more difficult than just spelling!

I do not think it should be divided this way.
Alternatively, we can compare writing and speaking on one side with listening and reading on the other side; i.e.
productive versus receptive language skills respectively.
I personally think that mastering the receptive skills is easier than the productive skills as the latter requires higher competence of the foreign language.