No language is the easiest

It sounds like you're looking for a single, objective answer. However, this is a fool's errand. To me, the easiest language is the one which you are motivated to learn, and you have speakers with a lot of time and patience to guide you properly.

It depends upon your native language because some languages are close related languages

I think that Mostafa is right. The hardest part of learning another language is learning the things that your language doesn't have. For me, the hard part of learning Chinese is pronouncing and recognising the different tones, because English isn't a tonal language. Also learning to read the characters is hard, because they are so different. So the relatedness is an important part of your question. So - what is simple? I guess it would have to be a language that (a) uses a roman script (b) doesn't have tones (c) has simple grammar (d) has regular grammar. Someone told me that Malay is quite a simple language. Does anyone else agree?

I used to think it would be Japanese or Korean, but once I started learning it, I found that it's hard.