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.
Turkmen is relatively easy to read and pronounce in my head as a Turkish speaker but man actually speaking it is impossible, to an average Turkish speaker Turkmen pronouncing sounds drunk and mumbled like your tongue is paralyzed