Which language is the most difficult to learn according to your experiences?

I haven't tried to study that many: Japanese, Mandarin, Cantonese, Swahili, and French.  The last one I haven't really tried that much, I mostly just thought about studying it.  In my experience, Japanese was the hardest.  The biggest reason I felt was because the word order is so different from English.  Also I found the different forms of speech for polite and familiar situations intimidating.  I never went beyond learning Japanese in the classroom so I only used the polite forms.  I couldn't imagine how much more time it would take to learn the casual style, plus there is even the extremely polite forms to learn as well!  Well, I do have friends who have studied Japanese and done quite well.  So it's certainly not possible.  But it definitely requires a lot of dedication.

In comparing the rest of the languages, I think that Cantonese is harder than Mandarin because there are less learning resources.  But both Cantonese and Mandarin have the advantage of no verb conjugations!  I also thought Swahili would be very hard since it's from a completely different language family as English.  But I was surprised by how regular the patterns are in the language.  Written Swahili uses Latin letters and, since it is relatively new, matches the pronunciation very closely.  Also I think my teacher's teaching style made a big difference.  He taught mostly through speech instead of written materials.  Even I only studied a short time, I could tell simple stories in the language.  Finally, I feel French would be difficult for me to learn.  This would be compounded by the expectation that it should be easier than Japanese or Chinese.  But in fact the writing is very inconsistent with pronunciation, there are many grammars point to pay attention to like conjugations, masculine/feminine forms, etc., and the similarities between Englisha and French can be misleading.