This question depends almost entirely (so I'm told) on which language is a person's mother tongue. That is, there's no "objective" or "universal" answer. It's a matter of perspective. A native English speaker, for instance, is likely to find Portuguese more challenging than a native Spanish speaker will. I'm personally finding Mandarin to be extremly challenging -- a lot more difficult than French. But as a native English speaker that shouldn't be surprsing given the fact that something like 50% of every day English vocubarly comes directly from the latter, and basically none from the former (moreover there's a lot of underlying structural similarity between English and French because of their common ancestry in the Indo-European language group). I suspect a native Mandarin speaker will find English a more challenging language to learn than, say, Catonese, for similar reasons.

Chinese :)

I think Chinese is the most beautiful language on the world.

Arabic

Arabic,because something about accent is diffeicult.