I have a pet peeve: job applicants who tell me in their cover letter that they are without any doubt "the best" candidate for the job.
This amazingly bold statement is often made by candidates who, in fact, match very few of the requirements of the job. But even if that weren't the case, come on. You don't know who the other candidates are, and (unless you're an internal candidate) you don't know the needs of the job intimately. It comes across as overly cocky, naive bluster.
I understand why people do this -- they've been told they're supposed to display confidence. But humility matters too (and it's rare that I've hired a candidate without some).