Convince the reader that you can write. Do this in your own style and one that matches your screenplay. Regardless of style, your letter should be easy to read, lively, and sincere. And, if your script is a comedy, make the reader laugh. Make it unique, but professional.
Professionalism is valued over kitsch.
You will hear stories of people who get through the door with outlandish capers, but these are more likely to harm you than help you. And, if you get through in this mode, you better knock 'em dead. Pitch your script. Give an expanded log-line. Pitch it with passion.