I think Korean is the easiest popular Asian language.
HAHAHAHAHAhhhhh and a few HAHA.
If you are captured by a foreign spy agency, then tied up and presented with two choices....
1- Water-Board Toture until you confess all the sins of the world.
2- Tie to chair and learn Korean
CHOOSE THE WATER BOARD TORTURE - it is way easier.
Having said that, Chinese, Japanese, and Korean present different dynamics throughought different phases of learnig.
Chinese - Damn difficult language to learn to a speaking proficiency, but you can get over some grammar right away and learn some words and function soon enough well enough at an early low level, but even that is a struggle. LATER, the learning never stops with vocabulary, stays a steep curve all the way to professor level.
Japanese - Danm difficult. Upside down. Grammar and structure simply different. You will not go anywhere anytime soon, but later, if you stick with it, can do some level up and get around.
Korean - Damn difficult right away. Everything has a grammar pattern and you damn well better learn the many types of honorific mods to verbs or you gunna get ur azz kicked. You could be learning full time studying only Korean 6 months and still feel like you would starve if you had to speak Korean to buy groceries or resturant. Getting the basics of grammar and some vocab takes FOREVER. 99.9 % of people simply give up. It is too much work and too much struggle early on and mid way. However, once you get used to all the grammar and can manipulate it, the language begins to make sense and is so natural you will think in it easily. You could even dream in Korean and go on TV and sing Korean songs like you Korean - they have shows for foreigner doing that. You better be damn determined and ready to stick with it or you gunna be a dropout failure.
Any of these languages requires structured training and reps and learning why you fail and a lot of early failure, so pick your poison. I would say for 6 months, you would go further with Chinese, but if you really want to get to a translator level of a spoken interview level (like say Adam Barbrow asks you to translate stuff to and from Seo Hyo Won) you better have a few years and chose Korean or Japanese. The learning at the mid and higher levels does not drop off with Chinese.