This is a question that many people have asked over the years. The answer is not as simple as you might think. While "best friend" is technically two words, there are some cases where it is acceptable to use only one word. For example, if you are referring to someone as your best friend in a casual conversation, you can probably get away with using only the word "friend." However, if you are writing something more formal, such as a letter or an email, it is better to use the two-word version. In general, it is always safest to use the two-word version of "best friend" to avoid any confusion.