Writing a business letter is about the content, not about jazzing things up with personality and panache. It’s a time to display clarity and concision, and that’s best done with a professional-looking, easy-to-read letter that follows all the established practices for letter writing. It’s typical to write a letter with just one font, but that doesn’t mean it’s the only font on the page.
When it comes to choosing the number of fonts on a page, the golden rule is, fewer the better.
In professional document design, experts will tell you to never have more than three fonts, because it starts looking chaotic and unsettled. “Chaotic and unsettled” are never things you want a colleague, competitor or customer to take away from any dealings with you, right? So, remember, if you’re using a letterhead, there’s likely to be other fonts in use in the letterhead; and if you're not using a letterhead, there's no benefit in having a second font.
Keeping the body fonts simple, clear and good-sized makes letters easier to read or scan. When choosing fonts, you’ll have to decide if you favor serif or sans serif. Historically, in books, Times New Roman and Bookman Old Type have been quite popular. These are “serif” fonts, ones with small lines or projecting features at the end of each stroke, and they’ve long been preferred as "easier to read" for headlines and lengthier works.
Sans serif are fonts that are clean with no embellishments. They’re considered very modern-looking and easier to read for beginner readers as well as people with disabilities. (Folks with dyslexia, though, now have a sans serif font that is very different-looking; bottom heavy, every letter designed differently and so on.)
The body of your business letter needs to be straight-up, not cutesy or personality-filled, so it’s important to stay away from fun things like Comic Sans, any kind of calligraphic script or style-heavy fonts like Papyrus.
Instead, pick tried-and-true clean type like Helvetica, Calibri and Arial for the body if you’re after a sans serif font, or a Times New Roman, Bodini or Georgia if you want something more classic and old-school. Whatever the font, the size should be 10-point or 12-point so that it can be read by people with average eyesight. Section headings can be in 14- or 16-point font, but remember to keep it classic and clean by ensuring there is white space around headings, too.
If you’re after creating emphasis, that can be done by using italics or bolding the text and even underlining for clarity. The rule of thumb is, use only one kind of emphasis at a time. Don’t use ALL CAPS because it’s considered “shouty.”
You might be sending the letter in email, in which case, it’s best to just stick with default fonts because they’re designed for screen-based reading.
For print, if you’re feeling vexed that you can’t put a punch of personality in fonts, know that the best way to stand out on business correspondence is in using a heavy-weight paper, like 30-pound stationery. (Traditionally, printer paper and standard business paper is 20- to 24-pound bond; buy heavier specialty paper at stationery shops, where you can feel the heft before purchasing it. Buy matching envelopes of the same weight if you’ll need them, too.)
Be sure to follow standard style tips regarding margin width, addressing and layout, so your letter looks professional. But most important of all, edit, edit and edit some more, so your letter is aces grammatically while employing great syntax.