Unpopular opinion but: XY/Z was a fantastic series. Kalos League was horrible. Sun/Moon was a decent series. Alola League was good.
I know we all wanted Ash to win the Kalos league because XY/Z hyped him up as being such a competent trainer with a sakuga Greninja on one side and doe-eyed love interest Serena readily awaiting him to sweep her off her feet on the other but I personally prefer the storytelling behind the Alola league. I didn't like Sawyer or Alain as Ash's rivals. They were both established as such really late because the writers remembered that Ash was supposed to have a rival only after half the series was already over. Hence, a lot of their development in the rival role was rushed and peeved me off. That doesn't help considering almost all of Ash's battles in the league were skipped except for the ones against said Sawyer and Alain so those two characters were the only ones who got any focus. So essentially nothing about the league engaged me in any way past the lowest common denominator known as hype. Kalos League was a huge disappointment for me.
And yeah, I think the Sun/Moon anime could've done better with how they handled this league. But seeing moments like Lillie or Mallow or Lana who we've known since the beginning of the series get time to shine, Guzma's character arc, and that very satisfying Ash vs Kukui 6v6 bumps this league above Kalos for me. Once again, it's just my opinion and taste. I prefer good characters in a lighthearted setting over badly written characters in a serious/dark setting. Sure the 1v1 and 3v3's suck but it makes sense why the writers would want to do that so to allow the league to pace itself better from a narrative standpoint. You can't even say it's because they got lazy to write/animate battles because this league is way longer than any previous one before it and doesn't skip a single match. I want as many 6v6's as the next guy but with how much attention the writers put into each battle, that's only possible if we got two to three parters for each one which just isn't realistic. But of course this league has its flaws. I could've used for better buildup to the league from the beginning of the series, better utilization of emotion on Ash's part, and maybe stricter rules on who makes it to top 16 to name a few examples. In all honesty though, I'm satisfied with his champion title. Ash taking out Kukui to solidify that it wasn't a fluke helped his cause a lot.
Anyways, bottom line is that Ash should've won in Sinnoh. If I felt like embodying the salty XY/Z fandom for a second.