I used to think that Burning Starfall and Majesta would argue most often.  
After all, one is directly similar to Rainbow Dash, the second to Rarity…
 
 
And then I thought… Starfall probably likes fashion too, but she just has a different style - something bright and eye-catching… They would argue just on the topic of fashion, and not “are you again with your hairstyles and outfits picking on me? Who needs this at all???”
 
And for sure they would both love selfies :D
 
(Now they remind me of my classmates from middle school, ahahaha)