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.
All of them I bet, provided the writing system is close to English.
Spanish orthography is very close to English, and I understand a good bit of it when reading/writing/speaking, but trying to decode anything audio at full speed is super difficult. I’ve had some exposure to German and French, and feel similarly (tho I don’t have the experience to say I can really read and write those).
Having learned enough Japanese to read easy material, I feel like listening is still the big hurdle. Wouldn’t say the written part is easy by any means, but it feels much more approachable when you can go at your own pace.
A great deal of it just comes down to how you practice. Since writing doesn’t have the built in speed component, I think that tends to feel easier and people (at least me) gravitate toward it. The real strat would be to learn mainly by listening, but it’s just tough to prioritize it.