Perhaps the one you enjoy most first, since the first one may take longer since you’ll also be changing and adapting your study plan to work best for you. The other languages will be faster to learn at least a little bit, because you will know your preferred materials/plan and know how many hours of study per day to hit milestones as fast as you want to.

Personally I did Japanese at first for 2.5 years and only learned N5 level material, maybe a touch of N4 material. Made a lot of mistakes like barely learning new words or immersing until 2 years in, also only studying 30 min to 1 hour per day, trying to study kanji for 2 years before just going ahead and trying a vocab approach instead. Everyone’s different, that just worked better for me. Eventually gave it up because it was progressing so slow and in 2.5 years manga with a dictionary were only finally barely comprehensible.

Switched to Chinese, worked much better. I studied French before Japanese so I applied the same study plan to Chinese. Japanese took 4 times as long to hit milestones as French (combination of bad study plan and not enough study hours per day). Chinese only took 1.5 to 2 times as long to hit milestones. I’m almost 2 years in and in some ways my Chinese is about equal to my French at 2 years in. I studied Chinese on average 2 hours a day which helped speed up milestones. Also I personally find Hanzi way easier to learn because they usually only have one sound (compared to Japanese many pronunciations per kanji), often have a sound component and meaning component hint within the Hanzi, and compound words are often somewhat to very intuitive based on Hanzi meaning. So once I learn a Hanzi in a word, I knew it and it made new words easier. Hanzi was just so much easier to me than kanji and still is. And Chinese grammar to me was much more intuitive and only took a handful of months to get used to passively comprehending. Versus Japanese where after 2.5 years I could not process grammar at a reasonable speed.

I’m finally going back to Japanese study too now, and kanji are eons easier because now that I’m studying the grammar again I can comprehend the general grammar (slower than Chinese but I get it) and kanji meanings/word meanings are much easier for me to figure out. The hardest part now is learning kanji pronunciations in different words, learning kana only words, and getting grammar more internalized so it doesn’t take so long to comprehend. And general listening skills, since right now I can read somewhat way more but don’t know how most things are pronounced. So I think Chinese study helped make Japanese more approachable for me. I imagine the same is true in reverse, so studying Japanese or Chinese should make the other a bit easier to pick up.

I have glanced at Korean a little but have no big desire to learn it yet. It’s grammar I’ve heard is similar to Japanese, so Japanese knowledge may help learn it - or vice versa Korean knowledge may help you with Japanese a bit. I do find with my Chinese I can hear some similar-words in Korean sometimes when I watch shows. Not sure how many near-cognates to Chinese there are in Korean, but enough that I have found books on the topic.