Best font for a printed formal letter in Word 2013 (or LaTeX)

Hello! I am looking for the best font to write a printed formal letter to a respectable doctor (a professor) I have never ever written to. It must leave a good impression since he is very bussy and would surely dismiss a poorly written letter. I have the letter written in Word 2013 (Calibri). I can rewrite it in LaTeX (probably using LyX) if necessary. I prefer free fonts but I am willing to pay for a good font if necessary. Thanks for help.