I saw this linked on someone else's blog (365 letters, I believe) and I thought it was a cool idea. As we know, I'm not great at following through on these, but I'll give it a go.
Dear Best Friend,
You have more than one name. You have more than one face. And yet how I love you. My life is forever changed because you walked through it. How I treasure the times we have laughed together, gasping for breath with tears streaming down our cheeks. You are the cheese to my macaroni. Please believe me when I say that you are a blessing from God, and I thank Him for you often. I love you. Ps - let's go cause some trouble ;)