Everybody is going to be different, there is no official criteria, but what language, and what aspect of that language, was most challenging for your brain to get a handle on. Also mention what your native language is.
Right now, German is really beating me up for me, a native english speaker, which may sound surprising.
Everyone says german is easy for english speakers because it is closely related, but it hasn’t been the case for me. I have actually found it more head scratching than japanese. I don’t know why, but my brain just likes japanese. It seems so logical, concise, and straightforward. I felt like i was able to pick things up from the context much quicker despite the grammar. And the fact that the verb conjugation is totally REGULAR and non gender oriented make it so much simpler.
In german, cases, sentence order, the idiomatic/ambiguous nature of expressions. The fact that that literal translations are never useful for most all of expressions we have, despite many words being similar sounding. It is very difficult to interpret and think the way a German would. I imagine this isn’t unique to german and many languages are like this, but it has been giving me the most headaches. Arabic was probably harder overall because of the time it took to learn the pronunciation, the script, the way their sentences work,(it took me a long time) but it hasn’t been this befuddling to me comprehending once you get to full speed listening of content. It’s just unexpected because german typically comes up as one of the easier languages to learn for english speakers, it hasn’t been the case for me.
What was the hardest language YOU have learned? It seems to be that most people say it’s their first language or their first non indo europen language. Chinese and Arabic typically come up as the hardest, and some people say japanese but that’s because of the kanji.