It depends on what language you are starting from! If you know Cantonese, then Mandarin won't be as hard as other languages. If you are starting from Spanish, Italian should be on the easier side. It also depends if you take into consideration the writing systems and/cultural aspects. Japanese, although the pronunciation is fairly easy, is supposed to be the number one hardest language to learn for Americans according to the Foreign service because it has more than one writing system including a pictorial one but mostly, the deference and cultural humility imbedded within the language is considered very difficult for Americans to master. 

My personal opinion is that the easiest language to learn is the one you love and really want to know and the one who someone you like a lot speaks so that you have motivation. The hardest language is the one you are forced to learn, even though you don't like it and don't want to know. 

Its very hard to generalize.

 I just think, there is no easy language to learn.

I imagine that the difficulty of a new language depends on what one's native language is. Writing systems, grammar, idioms, particles, and word order that are very different from a learner's native language would naturally make the new language more difficult. I've heard that for English speakers, Chinese is one of the hardest to master. I imagine that Japanese, Korean, and Russian would present some of the same difficulties. But for a native Chinese speaker, I'm sure it's hard to switch from writing characters and speaking a tonal language to an alphabet-based system where tone doesn't change the word and its meaning but could change, for example, a statement to a question.

I don't think the difficulty rating is as important as one's passion to learn. Just persevere and give it your best. And don't be afraid to sound like a fool as you practice your new language. We learn through our mistakes.

I believe there are no more and less difficult languages) any language has its own difficulties. In Russia a lot of peole consider that the most difficult is Chineese) but I know people who learn Chineese and English at the same time and they say  sometimes English is more difficult (: languages has many sides that's why choosing one the most difficult is wrong.

 I totally agree. I think language is changing all the time. Simfied and new words etc.  And I think interst is a quit important factor to decide whether you can master a language ot not.