I think it's important to mention the perseverance factor.
No matter how "easy" the language, if you don't stick with it then it will never be "learned".
The example being, my mother language is English. In theory, languages like Spanish, French and German should be the easiest for me (as they are some of the closest languages to English.)
However, I have very little interest in these languages. I don't really have much of a reason to use them and so I would probably give up along the way. Instead, I chose Japanese. A language which is one of the most different (and by extension most difficult) for native English speakers.
Sure, it will take me longer and it's more of a challenge but the fact that I'm more interested in the language and the culture means that I am more likely to stick to it (as I have done for the last 1.5 years) and eventually become proficient in speaking it.