Please stop opening your cover letters with this sentence and all variations of it:
“I’m applying for this position because I’m the best candidate for the job.”
No, you’re not. The best candidate has the sense to know that she has no idea what the rest of the candidate pool looks like.
Somewhere along the way, someone told you to be confident and sell yourself in the job application process. But that means showing me what makes you the best candidate, not just telling me and expecting me to believe it. If you’re the best, your accomplishments are what will make that clear.