Such an interesting question! Honestly I think the level you want to learn a language to is a big factor - if you want to learn enough words and phrases to survive a trip to one of these countries, then Japanese has to be easiest for me. Plenty of loanwords like:
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Beer: ビール Bīru
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Coffee: コーヒー Kōhī
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Toilet: トイレ Toire
Plus the pronunciation is far more straightforward. No dealing with things like Chinese tones or Korean vowels.
However, if you want to learn one of them to near fluency, I’d also agree Mandarin is easiest! Once you’ve got the hang of tones, how characters are constructed etc., the grammar is very straightforward. E.g:
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Verbs don’t get conjugated
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Nouns don’t change depending on gender/number etc
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The sentence structure is similar to English and follows a subject-verb-object pattern, unlike Japanese and Korean
Another advantage to learning Chinese instead of Japanese is that most (not all, but most!) characters are only read in one way and are only one syllable. In Japanese, manyyyy characters have multiple readings, which I find confusing.
But ultimately, the easiest language to learn for anyone is the language they enjoy learning the most, as it will be easier to stick to in the long run
If anyone wanted to read more about why Chinese really isn’t that hard to learn, here’s a blog post you might enjoy 