This, Everything this.Iris, no doubt.
Dawn's victory over her predecessor was a little jarring, but as we had already discovered from her stint in the show, May is definitely not unbeatable and was prone to making many mistakes of her own. Plus, Dawn had fallen into a deep depression after her last two contest challenges had ended in disaster. It made her much easier to root for and a far more deserved win.
Iris, on the other hand... ugh, never has there been a more forced and cheap victory. The problems with her team vanished into thin air just to let her win; Emolga suddenly obeyed orders without question, Axew learned the strongest move available to it to at the best time possible, (despite having so much trouble with the lowly Dragon Rage not long ago) and whilst Excadrill does indeed have a huge advantage over Electric types, Ash was practically forced to use Pikachu. And after 40+ episodes of Iris belittling Ash, she was asking for a Break the Haughty moment; the last thing she deserved was to defeat him and everyone else in a major tournament. Even Trip winning would've been a better outcome.