Kate and Sun? Are they even? haha... Also, I'm a little sad I didn't see the post to do suggestions because Carrie & Miranda (Sex and the City) are totally the understood best-friends of the series, not Carrie & Samantha. They would have been high on my list.

And even though they don't show it enough on screen (though it's obvi implied), I love Kara and Helo together as friends.