Formal letters should generally not have a catchy introduction because you want to sound professional. In general, you want to address a formal letter with "Dear Mrs. Romero" or "Dear Committee" rather than "To whom it may concern." If you can't find the name of the person you are writing the letter to, then you should say the name of the department or company. In an informal letter, you can use the first name and "Hi" or another informal greeting.
The opening sentence of a formal letter can either be a compliment, the reason for your letter, or the main point of the letter. Generally, humor works best in informal writing and getting to the main point is better in a formal letter. The most important thing is to be clear and make sure that you don't confuse the person reading the letter.