@Latecomer
Outside of the two-parters, where they almost always look bad, there’s nothing going on to make one seem better than the other (outside of the lack of a second throne until near the end of the series).
@Meanlucario
Well as you’ve already noted, most premires and finales. Also Slice of life, Princess Spike, the movie… to be fair, it’s nov so much that Celestia is necessarily dominant, as much as they share vhe “leader of Equestria” role she previously held alone. Heck, sometimes Cadance is squeezed in there to make a whole “other Princesses” package.
@Latecomer
Again, two-parters don’t count since they’re both used as the damsel in distress, Slice of Life was them bickering like siblings, Princess Spike is a Spike ep so quality is in the drain, and I never seen the movie. And them sharing that role makes them equal, not one an accessory unless the show shows otherwise, and with the number of ep showing how great Luna is, that isn’t the case.