Rarity's majesty is undeniable, but the numbers don't lie either
Yes, Rarity has won the hearts (and wallets) of untold millions of bronies with her looks, demeanor, atmosphere of fabulosity, and other amazing adjectives
However, Derpy won even more hearts (and many, MANY more wallets) with just a cute face and a love for muffins. Her untapped potential is simply immeasurable
Rarity is not a bad pony*, but Derpy is simply that much higher*unless you're Russian, because Rarity is pruportedly a walking faux pas to the Russians
Ultimately though, "best pony" discussions don't make much sense to me because the show has a pony for EVERYONE.
There is no one best pony, just best ponIES. [Merged your posts. Please try to refrain from double posts in the future, you can always edit your own posts. Thank you. -Davroth]