Well, you could try telling her what you feel (since you are already best friends), although it is best to first think how she may react to it. If you declared your love to her, and she were to say no, do you think things would get awkward between you? If you think so, then maybe it is best to wait a bit longer before saying anything loud and clear, but if not, just try to find the moment when you can talk to her properly.
As a personal experience, I used to like my best friend too. I told him and well, though he didn't like me back, we are still best of friends - so it is not always true that these things ruin friendships, in fact, on some occasions they just strengthen them more.