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 assume this applies to most of the east-/southeast-asian tonal languages (is Korean tonal, too? not sure). It’s just completely alien from the perspective of European languages. Though some writing systems are probably easier than others, Korean seems well-thought out compared to the ones based on latin script but covered top to bottom in diacritics and whatnot, and Chinese and Japanese reading/writing suffer from the existence of kanji.