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.
Spanish. I prefer to speak slowly, but reading and writing is rather easy for me. Having impaired hearing makes it quite difficult to understand native speakers, though. But since I had to study Latin in school, it helped me a lot learning Spanish.