Dreams are reflections of your feelings about your waking life. Dreams can be literal and/or symbolic. The trick is to figure out which it is.
Sex is a common symbol in dreams, but it can also be literal, reflecting a literal desire for sex. In the case of the latter, the next question is whether or not your partner in the dream is the real focus of your sexual desire or the person with whom you feel closest, thus making him the "best" partner in your dream... or a bit of both.
Check out the links to the definitions below to get more clarity about how recurring dreams and the symbols work...
sex
having sex
recurring dreams
Another clue... feelings in dreams are never disguised, so how you feel IN the dream, is how you really feel about the subject of your dream (having sex with your best friend) in waking life.
As the dreamer, only you can validate the meaning of your dream. Once you do, it's your call as to what to do with that awareness. Taking a best friendship (or any relationship) to a sexual level is not a step to be taken lightly. If you haven't already seen it, check out "When Harry met Sally"... or, if appropriate, watch it together.
Hope this helps. Good luck!