Greek, it turns out, is not the universal symbol for unintelligibility. The following chart traces the languages referenced in phrases corresponding to the English "It's Greek to me" to express an inability to understand in various other languages.
I personally did not need the proof of social consensus to convince me that Chinese is the hardest language but the hierarchy is pretty fascinating. (You can find some research on this subject here. I bookmarked this article twice in the last few months so it must be good.) I might also add as evidence my personal experience at a recent party, where, in an impromptu bit of amateur linguistics, a Spanish friend and I verified a good part of this chart. (I am not so sure about the Heavenly Script bit, however; the Chinese student we spoke with seemed to have no clue what we were talking about. All of us, including him, were giggling uncontrollably as he tried to guess what we might be getting at when we asked him to what language he might compare something he found incomprehensible. "I would call it difficult! ... No? Confusing? ... Ohh... mathematics!")