The one you don´t want to learn. I think that every language is "learnable" because millions of people speak their native language in a easy way and they learn that language since they were babies. Many people have to learn a language because of the necesity of getting a diploma in the school or the university rather that the desire of speaking or communicate their ideas with people who speak that language, other are obsess with perfection and they think that making mistakes is terrible and they need to learn all the grammar rules, all the vocabulary, everything about certain language. If you follow the same aproach or at least something which is similar to the way you learned your native language you will learn every language you want to learn easily and enjoying the process, you don´t need to speak your target language perfectly because even in your mother tongue you make mistakes, you don´t need to know all the rules and all the vocabulary. 

generally Chinese is on the 1st place and Russian is on the 2nd one. But all depends on your motivation. If you are not motivated enough and don't want to master the language, u will not have success with any language in the world.

Maybe chinese or arabic, it must be difficult to learn a lenguage with a different writing system.

I think Basque (Euskara) because it is not know by many people and because it has 24 cases to learn. It doesn't just change the ending of the verb it also changes the begining and it has many words that add prefixes, suffixes and infixes to simpler words to mean something else. Lastely it puts the subject, direct object and indirect object onto the verb itself unlike many other languages. There is much more to it this is just all the things I know.

The further east you go, the harder the language, also the older languages can be difficult to learn at times(Arabic, Chinese, etc), but it also depends on which language is your mother tongue and what other languages are similar to that: Spanish-Portuguese // Russian - Belurusian // English-Dutch // etc