Miley Cyrus known as Hannah Montana has a song called true friends which is about what true friends/bff's are about and what they do for you. Its an upbeat song that was one of her well known songs. 

[Chorus 2]
A true friend
You're here till the end
You pull me aside
When something ain't right
Talk with me now and into the night
'Til it's alright again

[Bridge]
True friends will go to the end of the earth
Till they find the things you need
Friends hang on through the ups and the downs
Cause they've got someone to believe in

The reason why I chose this song is because it has a good meaning to it and because it has a nice beat to it. Basically, this song gives you a list of things you should look out for when you have a best friend.  Though out this song it uses ethos because it's building up the character of a best friend that one should expect. 

Montana, Hannah. "True Friends." Letssingit.com. Ed. Victoria Ellison. 2000-2012.

 <http://www.azlyrics.com/lyrics/hannahmontana/truefriend.html