The letter format should use an appealing font. Maintain uniformity of font and size all through the body of the letter. At the same time, you should create headlines and titles to seem different by simply increasing the font size slightly. Capital or bold letters should be done with care so that reading continues to be smooth. The art of sales writing is time consuming. You could refer to a few sample letters to see how they have made good use of fonts to give a perfect finish.
If you want results for your efforts, then what you really need is to understand the most critical aspects of the cover letter. The format is just one aspect. There are more details that you have to take into account. The content is the most essential point you have to bank upon. You might have a very crisp paper with nice format in it but it will not be helpful if you do not indicate the skills and credentials required for a certain job position. Of course, you need to possess the same essential skills, traits and knowledge. Thus, you must evaluate first your qualifications before aiming for the brightest cover letter format.