To me, hanzi/kanji don't seem as hard as people say. It's just a matter of quantity really, it takes a lot of time and patience. But when I see something like Arabic or Thai, it seems someho harder. It's just a long line of tiny characters thar all run together, and it makes me think "lol what". It's less to learn, but I think it would take me an inordinately long time to get familiar and comfortable with that kind of writing system. That is just me though, I don't know how many other people feel the same.

I agree with tones making Chinese very difficult, but the relatively simple grammar makes up for that a little, especially when compared with some other languages. I suppose it's just a different kind of complexity, so it's going to vary depening on how your mind works.

The hardest overall must be a language isolate, if only because it wouldn't have connections to any other language, making it equally hard for no matter what the learners native language is.

What about African languages though? I suppose none have been mentioned because knowledge of them isn't so common. I know there are a lot of tonal ones, and of course they come from whole other language families, so some must rank pretty high in terms of difficulty. And the same goes for a lot of other regions of the world. It makes me feel very small when I think about it; all I'm knowledgable about is European and East Asian languages, and a few random other things. There's whole language families that are just totally alien to me, and probably most people in this thread, since on the internet we mainly focus on Europe and Asia.

I just had a vaguely existential moment there... it makes me wish there was more time in the world to learn about these things.