There is a reason many cover letters don’t get read. One reason people consider them a waste of time is because too often they are poorly written, mundane, not unique, they say what everyone else says. They are addressed to “To whom it may be concerned”, they don't attract attention. They ask for an interview in a way that sounds more like begging, almost apologetic for bothering the reader. They don’t display confidence. They don't establish rapport.
Write a unique cover letter for each job you apply for. One letter cannot fit all jobs.
It is a business letter. Make it a formal one.
Address it to a person by name, preferably the hiring manager (the decision-maker). Make an honest effort to find out who the hiring manager is (there are many ways). It is rare you cannot get the name of the ranking HR manager at the very least.
Grab attention quickly. State the logical reason an interview should happen, choosing compelling words. Do not ask or beg, call to action.
Demonstrate confidence; do not use words like “I think”. Demonstrate that you know what you are saying.
Research the company and the hiring manager. Establish rapport and align yourself with them by telling them how you fit in with what they are doing, or their company philosophy.
Close the sale. Establish that you will call them if you do not hear back in some specific time-period . Tell them up front so they will expect it! That makes it easy to get past gatekeepers.
Do not forget your manners. Thank people for their help and attention.
Sign "Respectfully", your name.