What language is the hardest to learn?

This question is VERY debatable and every language is unique in their own manners, however we have been able to develop a system in which we can categorize languages by difficulty levels and believe it or not English is one of those Level 5's (Hardest Level), if you are not a native English speaker. If however you are a born English speaker than there are three languages that are classified as the hardest for us to learn: Mandarin, Arabic, and Japanese. Mandarin and Japanese are hard because of their difficult writing system and grammar, not to mention its somewhat hard to read and they have no alphabet. Arabic on the other hand has a uniformed writing system and they don't use vowel sounds, so when trying to read texts the comprehension may be difficult for someone who isn't very experienced in the language.