Which are the most difficult languages to learn?

The world have right now a lot of languages, we know, but which are the most difficult? That's easy to know, but you have to have a perspective, a angle. Is not the same thing learn russian for an Ukraine as for an english.

But, still so, you can search languages particularly difficult to learn, like arabic or spanish. Because the spanish it's difficult, without doubts, and whether you're a english native, you know what I talking about.

So... this is my top five:
1) Arabic
2) Chinese
3) Russian
4) Spanish
5) French