Well, I don't think there is any language that is the easiest one in the world. It depends upon how quickly the learner can learn it. And, I don't think anyone would learn a language just because it is regarded as 'the easiest one!' There has to be a motive to learn the language. For instance, if I'm planning to move to a certain country, then I would have to definitely learn the particular language no matter if it is easy or tough.
But if I'd be planning to move to Asia or to Europe, then I'm sure I'd have many options to choose as to which country to move in and which language to learn. In that case, my advice would be simple: learn English, Chinese and Hindi. Chinese is the toughest language among all. But if you learn these three languages no matter how easy or tough you find it, you will find it very easy to interact with most of the people because these are the three most commonly used languages over the world.
