It seems to me that the languages I enjoy listening to the most are the ones I am most interested in, therefore, I learn them quicker. If certain words catch your attention or if you're naturally drawn to a certain language, you can start there. Another way to look at it as far as easy goes, is to try to learn languages that are closest or most similar to those you currently speak. For example, if you speak French, it should naturally be easier to continue learning one of the other Romance languages. Learn what comes naturally.
