I think all of languages are difficult except your mother tongue because if you wanna learn a new language u must learn the language culture too, I mean you have to know how you can use the words and how to put it in the right sentence therefore learning language besides culture is essential. 

I think that it depends from your native language

In my opinion, the most difficult language for me is the language that I am not interested in now and be forced to learn it! 
No interest, no passion, no feeling about this language will make me bored quickly and give it up soon

Yes, I agree with Mehmet. Russian is my native language and I can say that our grammar is difficult even for Russian students. However, we have one really important advantage which makes our studying process - almost always we write words how we say them. It is good advantage for easy learning.

Thank you for all comments. Many people say that Chinese is one of the most difficult language. What do you think about it? Personally I think that every language can be difficult except your native language but it also depends on our wishes and practice. If we have goal and we reach it problems which appear in the our way can be overcome.