Yes, I have heard, correct me if I am wrong, the three hardest languages in the world is:
Chinese-Mandarin
Russian
Arabic
It depends on where you're coming from. These are three of the hardest for English speakers to learn, but they would be quite easy for a Cantonese, a Serb, and an Ethiopian to learn.
For sheer difficulty, I've heard that Finnish, Hungarian and Georgian are even tougher languages.
Finnish and Hungarian are both Finno-Ugric languages, and Georgian is practically an isolate, having only a few very small relatives. What they all have in common, though, along with most Caucasian languages and the Turkic languages, is that they are all highly agglutinative, and therefore extremely regular. They have few or no irregular verbs or nouns. For a speaker of an Indo-European language, they are going to be somewhat difficult simply because they work in a different way and have little shared vocabulary with IE languages, but looked at objectively they are some of the easier languages to learn. They don't have the stereotypical IE features of gender, multiple declensions, irregular conjugations, and sound mutations occurring when word endings change.
