Sob, this has potential to be insanely rambling and repeating the same things over and over kind of response. What's there for Brennan to say about her BBF (no, I'm never letting Brennan forget this lol) that hasn't already been said? Angela was the first person who actually cared enough to look past the thorny surface of awkwardness and bluntness and strangeness to the person Brennan is underneath all of that baggage. Brennan feels comfortable around Angela to be herself without fear of being judged, even though she knows she may sometimes stretch Angela's patience. Brennan knows she can trust Angela unconditionally, whether it's just something simple as explaining certain things or something serious as providing back-up with her mad purse-whacking, eye-spitting skills. Besides Booth, Angela is the only person Brennan would drop everything for to help. Let's face it, Brennan was really rather lonely before she met Angela. She needed someone to drag her out of the lab to live life to the fullest. And to do jello-shots with.

Essentially, Angela is the sister Brennan never had and she ♥ her very much.