No such thing, it depends on your native language, what other languages you've studied, and a whole bunch of other factors.

Like Sethe said, it depends on a number of factors. To me, it also depends on your taste. You'll hear lots of people say that Mandarin Chinese is one of the hardest languages to learn. After studying it for several years I can say that it generally isn't once you get over the tones and characters. To get to the aspect of taste I mentioned, what made Mandarin tough for me were the sounds. They sounded plain weird to me and whenever I spoke it in the past I felt like I was making funny noises.

Any language is more difficult instead of native, you will always count that any other language is hard 'cause you have mentality that is opposite of the foreign language. For example English language is well-know as easy speaking and recognized by the world community as international language. But it is really hard for Russian people and Japan people in learning, 'cause ours languages is more wide in lexicon. Eastern and Greco-Slavic languages is said to be more difficult than other, in this list are Russian (the most reach of all listed), Arabic (too hard), Chinese and old Greek (the biggest language in the world for lexicon). You need to derive criteria to definite the most difficult language in the world. As for me the single criteria of complexity is pronounce where English have top position.

Grammatical rules are also a part of a language which makes that language makes complicated and programmatic. I saw many languages are full of grammatical terms and conditions.

Japanese