Top 4. Because there's classical Chinese (wenyanwen).
Whereas modern Mandarin is merely perversely hard, classical Chinese is deliberately impossible. Classical Chinese really consists of several centuries of esoteric anecdotes and in-jokes written in a kind of terse, miserly code for dissemination among a small, elite group of intellectually-inbred bookworms who already knew the whole literature backwards and forwards.

原因四：中文里面还有古汉语（文言文）。
如果说，现代汉语再难也不过是让你无所适从而已，那么古汉语就简直是故意让人永远也别学似的。古汉语确实含有数千年来积累起来的各种宫廷秘闻，用词简洁扼要，简直象某种密码，只适合那些天生的知识精英，他们对文学的一切了然于心。