For me, it has to be Korean (Hangul is easy to grasp) but the pronounciation, my god: it’s like I can’t understand a word being said when they speak (while that part is manageable in listening to spoken Japanese). Maybe it’s just me not having the “trained” ear on listening to Korean phonology in real time whilst learning the language.
I am not sure, only speaking 4 languages at least to some minimal degree.
French pronunciation sometimes drives me crazy.
Some of the sibilant sounds in Slavic languages are also really hard for me to get. But then again, I am a native German speaker and still have difficulty discerning between “ch” and “sch” in my own language, so perhaps not that much of a good measure…