How to write a top notch cover letter?

Writing a cover letter is a hard mission, to me it is harder than the CV it self.
any way you need to cover these sides when you 're writing your cover letter: make it simple.
it shall cover all your strengths.
it shall answer the question of "why shall we hire you?" it must be formal but not tough or too friendly too.
you shall request to be hired but you shall not look like as you are begging.
don't write it from a template.
No Mistakes :)