This is just my opinion, but I actually really disliked the Flare Arc. A lot. It felt rushed, it made the league rushed, and we never got a Clemont vs Xerosic battle. I felt that it focused far too much on Alain/Mairin.
Yes, all the gym leaders and Steven and Diantha and Myer uniting were awesome, but to me, it really came down to an eight-year-old singing a song to a giant brainwashed serpent and then encouraging it to kill Lysandre the next episode.
As for Ash/Alain/Malva vs Lysandre, I feel like Ash's team totally got shafted for Wonderzard and Mega Houndoom. And I still have no idea what the heck he was trying to do with the giant rock and hanging Ash's team up on the side of prism tower.
TBH I feel like XYZ kind of fell apart after the Machine fair filler episode (aka before the league). Just far too rushed to finish before SM. But the buildup was great, possibly the best in the series.
That's why I loved Galactic. It had a fantastic buildup (almost as good as Flare) and a cool culmination. Not to mention that it was nice how a lake spirit each appeared to one of the main characters and bonded with them, respectively. As for Aqua/Magma and Plasma, well, Plasma could have been good before the earthquake, and Aqua/Magma started out well and then totally fell apart.
So, if you ask me, the order goes Galactic>Rocket (BW)>Flare>Plasma>Aqua/Magma>Rocket any other time.
As for Aether/Skull, I agree with Jango, but I don't think it'll go very high up on the list since Skull (by far my favorite team) has literally disappeared into oblivion