Your cover letter should be well-presented, concise, and to-the-point.
So use an easy-to-read font, and don’t get carried away with embellishments. No pictures, no Comic Sans, and definitely no word art necessary.
Aside from ensuring its written using clear paragraphs – it also should be the right length. Too long, and you’ll risk rambling (and/or boring the recruiter); but too short, and you’re unlikely to have covered everything. Aim for half a side of A4 (or one page maximum), and you’ll be on the right track.