Hmm... you're right, it's not a nice feeling if you think that all the best guys prefer your friend to you.
If I understand you, you say that your best girl "friend" spent a long time chatting on the 'phone with your own boy (dating) friend and you are worried about him deciding he prefers her to you?
That's hard to take, but sometimes you just have to be realistic.
It is not because you are not cool or not goodlooking
Maybe it is because your best girl friend is not such a good friend after all. Maybe she just likes to see if she can "catch" the attention of a guy for fun and she's chosen your boy friend. That's not really a very good way to be and not the sort of friend I would really want around me anyway.
If your boy friend decides he prefers her, there's not much you can do really; except perhaps realise that maybe he's just a little shallow and his head is too easily turned. But boys can be a bit ditzy like that. If you give some of them a compliment and flirt a little they think you're hot
Ugh... hormones
Be yourself. Don't try to "be cool" if that's not the real you. There are hundred of potential boyfriends out there, and if you are a nice person and generally good fun to be around and easy to get along with, those guys will notice you.
Try not to let other ppl's disappointing behaviour get you down and don't think that it's okay to act the same way as them.
Stealing another persons boyfriend / girlfriend is just plain mean.