I feel like it'd be an easy competition for Celestia. Just depends on how dirty she wants to get.

"Okay everypony, we are about to begin deciding who the best princess is! Now, let's start off with a question. I'll ask one first. ahem Has either of us ever done anything horrifically bad to our people and or country?"

dead silence

"Oh, well I know I most certainly haven't.' moves toward Luna. "What about you, dear sister of mine? Have you ever done anything horrifically bad? Awful, treeeeeaasonous things?" Leans in closer. Luna can feel Celestia's breath on her cheek. "Things like, I don't know, hostile takeovers? Things that may or may not have resulted in banishments of some sort?? Hmmm? Have you, Luna? HMMMM!!!???"

more silence

Random Guard: "Awkwaaaaaaard."

Yeah, but one death stare from Celestia would have Twilight running away in pure Godfearing terror.

Plus, Celestia made Twilight, she can easily unmake her. Same goes for Cadence, so there's no way she's even trying.