You should learn the language that you will be hearing the most. Because the words will sound more clear if have heard them in the accent they are being spoken in.

Scottish English is the best! I bet that they can't even understand each other!

I wouldn't worry too much about pronounciation, becuase you will be intelligently understood even if you speak with an accent. That being said however, do go for one which you will use and prefer! The differences are small. In addition, knowing one type of English will still enable you to understand the other type.

I wonder what Americans thank about this question. Why is there no one who is from the USA to reply it ?

Alright, I was just trying to be balanced with my first answer. I'll admit it, an English accent is best. Specifically, a southern English accent. All others are rubbish. haha! and I hate it when MS Word tries to tell me I've spelled something wrong because I've not used the American version.