I think you can have more than one 'best bud'. I also don't think it matters who she likes best. She obviously wants you both for friends, and you're both lucky to have such a friend. If any of you bring jealously into this friendship it could ruin it. I suggest you accept her friends as this is the best way to be a good friend. If they make her happy that should bring you happiness as well. I'm sure you all fill different places in her friendship dynamic and she would be sad to lose any of you. Whatever you do, never try to make her choose. Whoever does this will lose, and will hurt your dear friend.