I believe if Rarity was voiced by anyone else than Tabitha, she would be an awful character, likely either with a Valley Girl or a Brittish Posh accent. But Tabitha is a great voice actress and her hammy overly dramatic trans-atlantic accent is just perfect to the point where the character feels half like a parody of itself. She was my favorite from the start and has been my favorite ever since. Her episodes are always pure gold, though she often needs some other character to act as the "straight man" to contrast her hammyness. Applejack probably being the best example. Look Before You Sleep and Sisterhooves Social being great examples.