For me, Julia's audition easily.
When you sing a song as huge as ain't no way and kill it, it's gonna be hard to top that moment, and she never did imo. Also, post editing may have helped her out too since she never sounded nearly as good in other performances.
As for McKenna. Quite a few to choose from, but I'll go with both sides now. Without thinking too hard it's probably in my top 5 performances of the season.
McKenna: Make You Feel My Love. She had so many fantastic ones but this was my personal favorite.
Julia: Set Fire to the Rain though I loved every single one of them equally