It doesn't matter.

It doesn't matter which, you will be understood in any English speaking country if you learn any of them. Personally I would try and expose myself to different variants and accents of spoken English, I think it would be more beneficial than just restricting yourself to one kind. Personally I think people get too hung up on this, English has enormous variety in accents and for me there is no one 'best' kind.

Agree, the accent doesn't  matter. Plus if the English is not your first language, your English accent will be influenced by your mother tong, anyway.  As long as your pronunciation is clear and people can understand you and you understand them... Grammer and vocabulary are the most important.

American. Think of all the American films, popular worldwide.

It's been said the Mid-eastern American accent is the most pleasant. But it's really a matter of opinion.

 Neither slang  nor accents are a significant issue.

  To learn a  language proficiently,  you can do something as simple as  write   a single sentence in a notebook, and afterward,  write several more sentences and then  vary them slightly.

 It helps to  use the Dolch Lists for the most  100 common  English Verbs and Nouns 

and to use these in many sentences  with variation. 

Word associations  between Synonyms and Antonyms  can be of great benefit.

   A  language practice partner  can be very helpful,  but it has to be an activity that is something other than  Finding----A---Friend.   A  "friend" may have no knowledge at all about how to advance in a  language study.

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