In may case, all of them. ;-)
. . so the winner is . . Chinese !
No language is more difficult to learn. However, some languages take more time to learn. As long as you have patience and study wisely and regularly, you will learn it.
I have studied Chinese, French, Korean, Turkish and Spanish. I have achieved a high level of fluency in Chinese and am at an intermediate level in Spanish. My native language is English. Chinese took the longest to learn because the writing system is not phonetic, among other reasons. (Chinese is the only living language in the world that still uses a non-phonetic writing system!)
But, I enjoyed my Chinese studies so much, that it was never "difficult". It just took a much longer time than Spanish, for example.
If something is pleasurable, isn't it better when it lasts a long time?!?! ????
Spanish is difficult, because it is complicated and there are many details. Japanese, Hindi, Arabic, Chinese, also have details, but for me they are easier. It´s more related to my brain.
I've heard it's Basque, because there is no correlation to another language.