#1 Asuka vs. Carmella
After Asuka finally suffered her first WWE loss at WrestleMania, the follow up was crucial. That loss had to come at some point, but it didn't need to be fatal for her career. She could have gone into the next phase of her career with a new purpose.
What she got instead was Carmella.
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It was apparent long before it happened that putting the blue brand's women's title on Carmella would be a terrible mistake. Dull segments and an awful match with Charlotte at Backlash during her first month as champion confirmed this.
Asuka was next to feud with Carmella. James Ellsworth returned. Asuka looked like an idiot week after week, in segment after segment, culminating in the worst match of the year at Extreme Rules.
The worst feuds elevate no one and damage a performer's career beyond repair. For Asuka, this appeared to be the case, until a Christmas miracle occurred and she got a new push that finally saw her capture the title that long bedeviled her. It was because she remained popular in spite of this feud, and because of her new push, that she ends the year on a high note.
Carmella, meanwhile, ends the year doing dance breaks with R-Truth, which is where she belonged all along. This feud did nothing to elevate her game, either. It was just a classic misallocation of resources that WWE so excels at.