Out of the mane six? At the beginning of the series, it was Twilight. Her anti-social antics was something I never expected to see in this kind of show. But then she grew into this “I know everything” character. Sure she studies a lot but when you can just copy someones special talent, you’re walking in Mary Sue’s home town.
Its a split between Pinkie and Rarity though they both have their flaws. Pinkie and Rarity can both be annoying.
I say Pinkie because she’s all about making others as happy as she is. You don’t see that often. Rarity because she’s basically an artist.
Outside of the Mane Six? I really wish Sapphire Shores would have another speaking role. Besides her I really latch on to the non ponies for some reason. The mules, donkeys, cows, dogs, dragons, etc..,