In my opinion, it would have to be writing and reading. That requires the most time to master. Speaking takes some time to get used to when you are first practicing the pronunciation of all possible sounds and tones, but once you've finished that, it becomes second nature. Surprisingly, the grammar is also fairly simple. I've only ever had difficulty with remembering how to read and write Chinese characters.

1. memory a hudge number of Chinese characters. 2. Chinese expression logic is quite different from English. 3.understanding Chinese culture behind the language.

From my perspective,the most difficult part is how to pronounce characters correctly.because most of foreigners can't get rid of their noticeable accent in chinese even some of them have learned it for many years and it's not rarely seen .

i think the most difficult part would be writing the characters and the pronunciation :o

The most difficult is a variety of meanings of a word or sentence, and ancient prose.