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Diantha has a better design than Cynthia. The Kalos Pokemon League and the Radiant Chamber were better settings than Cynthia's, and Diantha's battle theme totally played the part well.
Diantha's design and inspiration, setting, music, signature Pokemon choice, etc. all came together for this grandeur presentation and it worked wonderfully.
The only thing Cynthia has over Diantha is her overall team, but XY as a whole suffered from difficulty issues when compared to Gen 4 so I leave that more to the games themselves than the individual characters. I can't even argue story because Cynthia was just a crummy exposition dump that didn't do anything and Diantha was just this looming figure that you saw and heard little of.
She could've actually been integrated into the plot due to the nature of Lysandre's schemes to create a new beautiful world, but Game Freak totally dropped the ball on that.
