Its like Derpy came back and brought legions of lost fans with her."
And my take on that is: Yes.
That's precisely what went on here.
She's the heart and soul of this fandom in my opinion, and evidently I am not alone.
She is like ... the metaphor for us all, whenever we feel silly and mistaken, she exemplifies the grace with which we wish we could accept that part of ourselves we once thought had been a flaw. Sure the cutie mark crusaders are great examples of personal growth, but a lot of us are already "grown up" and aren't happy with our place in life...
But look at derpy. she's happy, doing her best, giving her all, no matter what difficulties she seems to encounter. She doesn't let her clumsiness be the boss of her; she transcends the narrative of "overcoming" -- because there are some things you simply cannot overcome. She symbolizes accepting, and growing to become part of, those things that can't be changed.
And that, my friends, is why I believe derpy is best pony.