Mythical Pixie Pokemon
Competitive NFE/Evolution Pair
Giratina-Style Forms
Normal-Type CAP
For me, it was a tossup between the evolution pair and runner-up, as well as Giratina Forms and abilities. The Primal Weathers seem to be begging to restart a conflict in Gen V, and Flash CAP already did a Protean User near the start of Gen VIII, so I don't think another one should be done again.