I believe Korean is the easiest not only because I am a Korean but also because we use phonogram only. Korean alphabets, Hangul, won gold in world alphabet olympics.http://languagelog.ldc.upenn.edu/nll/?p=4253http://asiasociety.org/world%E2%80%99s-most-incredible-alphabet
Chinese is the hardest since Chinese characters are ideogram. However, I think it is the most useful language because of China's recent growing.
About Japanese, you also need to memorize some Chinese characters because Japanese also use the characters.(pronouncing the Chinese characters in Japanese way is different with Chinese sound though) Plus, Japanese uses two types of phonograms, hiragana and katakana. That makes it harder to study.
I know this comment is really biased because I am Korean hehe
but Hangul is said to be the most scientific and easiest to learn phonetic sound.
However, there are still many other reasons to consider before starting any languagues.
Regarding grammer structure, Chinese may be the easiest for you because a grammer structure of chinese is similiar with that of english. Korean and Japanese has a different grammer structure with english.
Think about your real reasons why you want to learn the language first. I recommend you learning their culture first so you can learn the language that most attracts you.
I think you can go to the nearby Korean church and take some Korean classes for free!