Swiftgaiathebrony 
Both great villains turned heroes but both with stupidumb backstories and reasons of becoming evil.
Sunset for being a pettybgitchrl upset of not being made a princess.
Starlight just because her friend moved away, which doesn't sound all that tragic especially considering she's shown to have a family in the Parent Map.
I'd go with Sunset mostly because her reason of becoming wasn't as dumb as Starlight's and because of all her great songs in Equestria Girls. But Starlight is still a great character and good student for Twilight.
Sunset for being a petty
Starlight just because her friend moved away, which doesn't sound all that tragic especially considering she's shown to have a family in the Parent Map.
I'd go with Sunset mostly because her reason of becoming wasn't as dumb as Starlight's and because of all her great songs in Equestria Girls. But Starlight is still a great character and good student for Twilight.