I have found the Romantic Languages, as they are sometimes called, French, Spanish, Italian, Portuguese, the easiest. They are for the most part all related to Latin and have a flow about them. For someone learning a second language I believe English (American or British) to be the most difficult.
For European language speakers, Esperanto is probably the easiest human language to learn. Mostly, that's because it has a lot of words common to their own languages, and partly because it was designed to be easy.
Chinese is the easiest in the world.Just need 3000 Chinese character,you can read news and article in different fields.Spanish and German are difficult for their declension of words.There are millions of words in English,among which haven't obvious rules!