أعط أجوبة - English

Depends on the language’s written or spoken aspects, what your linguistic background is and whether your are an auditory, tactile or visual person.

I would say that many aboriginal languages in Australia would fit the bill of ”most difficult”. Highly tonal and horrendous grammar, not to mention a huge number of cultural elements to wrap your head around. They become additionally difficult when you realise that men speaking to men and women speaking to women form essentially different dialects for many aboriginal languages. In some cases they are even totally different languages. So to ”properly” learn many aboriginal languages, you would actually have to learn at least one additional language.

In the scheme of things, Chinese is relatively easy to speak for many people if it is taught in a proper manner and you are an auditory person - with of course proper control of tonal pronunciation at a beginner level. The grammar is quite simple. However, its written characters require some work, especially if you are not so visual in your learning style.

Most languages - in the spoken form - are made far more difficult than they need to be because of a lack of work on speaking accuracy when they are being taught. This tends to be the result of teachers/instructors making things easy for themselves by focusing on teaching grammar by rote or word recognition with flashcards and so on - and not putting in the hard work of demonstrating and fixing up pronunciation and other speaking elements.

Cherokee, according to an old colleague of mine, is very complex both for its grammar and also tonality - hence the rather complex keyboard for writing out Cherokee in Latin characters.

The hardest European languages would have to be Finnish and Hungarian given the grammar of their multitude of noun cases and also pronunciation which deviates quite significantly from many other languages. I guess you could also add Estonian to the list if you consider it completely distinct from Finnish.

Chinese -- some dialects more than others.
Arabic
Native American Peoples’ languages, e.g., Navajo, Ojibwa, Aleut, Hopi, Lakota, Chumash and several more.

Just speaking from my POV,
easiest: English
toughest: Japanese

In my experience, Cherokee, Arabic, Navajo, and Chinese are the hardest. All of them require proper tonality which can be difficult for the non native speaker.