I think it has to be between Rarity and Flutershy, who seem to spend alot of time together and they both seem to understand fashion (fluttershy knowing crazy stuff about design in suited for success), and Rainbow Dash and Fluttershy, who have been practically life-long friends since the beggining in Cloudsdale.
I would of put Rainbow Dash and Applejack near the top but their competition and prideseems to get a bit in the way of their friendship even though their very similar.
But then again as we know fluttershy is very shy so maybe the reason she is seen with her friends alot is because she dosern't have many other things to do, although, this probably helps the fact that she is top with closest friends because this means she spends more time with her friends making them just get more closer together.