The answer is very easy and certain.
The hardest one will be one one you are less interested in, or the one you really don't love enough.
I speak Chinese, study Korean, and have just a little deeper than general knowledge about Japanese, so I would say that everything depends on the thing you find the hardest about the language. For me the hardest thing is grammar, so I think that Chinese in this aspect is easy and good to learn. Korean is much much harder for me. But if you have problems with pronunciation, so Chinese will be really hard, simply because without tones no one will understand what you say. And Japanese is a mixture of complicated grammar and characters which for some people seem to be the biggest problem. So I would answer Japanese.
PS. Cantoneese is really hard as well, but speaking about Chinese we usually mean Putonghua
I'm a native Japanese speaker and learning Korean and Chinese.
When it comes to the writing system, obviously Korean is the easiest. As for the other two languages, you have to learn tons of Chinese characters.
In terms of the pronunciation, Japanese is the easiest. In Korean, you have to distinguish three types of 'k', 't', 'p', 'ch', and in Chinese, you have to learn the four tones, which is really difficult if you don't speak any tonal languages.
But Chinese has the largest population, and there're a lot of Chinese in italki (I guess), it's the easiest to find someone to practise with.
It depends on which part you think is difficult.