Best pony has gone through many voices over the years, confusingly enough. Here they are from season one through to the Holiday Special.

Voice #1 is included mostly just for posterity. At the time it was significant to think that the voice speaking "muffins" is possibly hers. However, other background ponies have matching lipsync.

Voice #2 "Little Boy" is Derpy's big premiere in the show. At the time the VA was not told Derpy is a mare. [Source screencap: https://i.kym-cdn.com/photos/images/facebook/000/240/284/588.png]

Voice #3 "Censored" was Derpy's replacement voice to "censor" her ditzy appearance. Notice her eyes are also changed.

Voice #4 Was from the show's 100th episode. It's a much more "standard" pony voice but there is some unique pronunciation with the "Rs" aka rhotacism.

Voice #5, the most recent as of this upload, is from the hour-long holiday special. It's the same voice actor as in the past, but the rhotacism has been replaced with a slower pace of speaking as well as high rising terminal aka "uptalking" [sometimes mistaken as "valley girl" speak.]

All voices [except possibly #1] are performed by Tabitha St. Germain. My Little Pony belongs to Hasbro.