that is a hard question to answer because that could be any language that is not a romance language ( anything that's not Portuguese, Italian, French or Romanian) is going to require a lot of effort for Spanish speaking people to master. But your right, I think Chinese and Japanese are two of the most difficult languages in the world for ANYBODY who is not native to an East Asian Language. Chinese is just hard for everybody! For pronunciation Japanese may be a little easier because the pronunciation of the romaji is very similar to spanish ( with the exception of the R, because Japanese Speakers cannot prounounce R and L like Spanish speakers can)

I don't know how hard Hindi is for non-natives but I have heard it is pretty hard. It is a well accepted fact that Chinese takes the cake :D

Not just for Spanish speakers, but Mandarin Chinese is the most difficult for most people around the world. However I think there is an even more difficult one: Shanghainese! They have even more tones than Mandarin!

Chines, japanes, corean and arabic