There is no "original" English or any other language, and such a belief is based upon a fundamental misunderstanding of the nature of language. There was no "creation" of English, but rather a transformation from other languages. English has continued to change since, and both American and British English have been changing since the foundation of the colonies, and neither is necessarily closer to the "original" than the other. (It's actually not even correct to refer to any English in the singular, since drastic variations exist within each.) In many ways, American English preserves aspects of "original" English that British English has abandoned, both in grammar and pronunciation. Examples: http://www.pbs.org/speak/ahead/change/ruining/

So North Americans are not "ruining" English, and any preference for one language over another is subjective.

British accent is easier to understand for non native english. I preffer the British accent.

Canadian, obviously! lol

We Americans do get lazy in our pronunciations.  For instance, very few people pronounce my name correctly.  While it is Dev-in, most Americans pronounce it Dev-uhn.  I believe it is simply because it takes a bit more effort to pronounce "ehh" than "uhh".

I could go on all day with similar examples.

Yesterday I watched "Lost" in original and observed, that Americans "swallow" words, so it is sometimes really difficult to understand what they try to say. But there was no such a problem with Charlie or Desmond. 

Needless to say, I like both accents. Much depends for me not "how" but "who".