Everybody thinks different about this topic.. For me Chinese.. And for you ?
So far, Polish. I find it challenging in terms of spelling, pronunciation and absorbing the vocabulary or grammar. I learned 'wesolych swiat bozego narodzenia' (merry christmas) and my brain almost melted! I have attempted to learn over 10 languages and this is the first time i have struggled this much...
learning new languages is very interesting to me it all takes time but very pleasing i think the language i tried to learn and failed is heliographic i can't even pronounce it. But i,ll give it one more try when i retire :)
Mandarin classes were hard. I'm interested to see how Korean compares!
I think that languages without the native alphabet for the people who is learning it's really difficult. For example for ocidentals Mandarim is really difficult because all the alphabet is different.