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：中文里面還有古漢語（文言文）。
如果說，現代漢語再難也不過是讓你無所適從而已，那么古漢語就簡直是故意讓人永遠也別學似的。古漢語確實含有數千年來積累起來的各種宮廷秘聞，用詞簡潔扼要，簡直象某種密碼，只適合那些天生的知識精英，他們對文學的一切了然于心。