Believe it or not, Rainbow Dash used to be my number one favourite character from episode 1. Now she's just unbearable, and is probably one of my least favorites of the mane 6. It was until "Fall Weather Friends", when I realized I hate her. She is such a bitch. In fact her bitchiness can't be describe with simple blatant words. She picks a fight with every mane 6 member in every episode. Not to mention, she criticizes each of the mane 6 for whatever reason.
To Rarity in "Dragonshy": She gets mad for Rarity putting on her scarf and checking if she looks okay
To Applejack in "Fall Weather Friends": LITERALLY CHEATED and got AWAY WITH IT. Goddammit we need more Applejack lovers
To Twilight in "Fall Weather Friends": calling her "egghead" (that's rather insulting)
To Fluttershy: always comes off as abrasive to her.
To Pinkie Pie: She yelled at her in tanks for the memories for no reason, and often is blunt about Pinkie being "annoying" or whatever.
In conclusion, she may have some redeeming qualities and be loyal to her friends, but she's way too cocky and childish for my taste. Plus the other mane 6 are way more loyal than she is.