His BF version is overrated, his XY version is terrible, and his DP version was pretty great when it mattered. But his SM version is definitely the second best in terms of being a competent battler behind the DP version.
It’s like you guys forgot that he swept the Kahunas, won the league, and is now on par with a Champion.
Yes, he swept the Kahunas because they are
OBJECTIVELY weaker than gym leaders. Why? Because you're comparing the Kahuna with their Z-Moves to the Gym Leaders and their non-mega Pokemon. Brock's base Steelix is stronger than Olivia's Lycanroc with its Z-Move. Kiawe beat her despite having a type disadvantage, yet lost to Brock the moment he Mega Evolved his Steelix. Even when Brock battled Olivia, he used Sudowoodo rather than Steelix because both know fully well that base Steelix would sweep Olivia's Lycanroc despite its Z-Move, given that Olivia is weaker than Kiawe who brock beat at a disadvantage.
Also, how can you think BF Ash was overrated and XY Ash was a terrible trainer when both are far more competent than SM Ash. In terms of competence: XY Ash > DP Ash > BF Ash > AG Ash > Johto Ash > SM Ash > BW Ash > Kanto Ash.
What you don't understand is that SM Ash swept the Kahuna because they are weaker than Gym Leaders. A big reason why DP Ash lost a lot despite being highly competent was because the DP gym leaders were significantly more challenging than the Kahuna. Do you unironcially think SM Ash without using Z-Moves could beat someone like Volkner or Wulfric (with or without Mega Evolution), or even merely Juan? This isn't even getting into the likes of Paul, Sawyer (without Mega Evolution), Tobias (with or without Latios), or Alain (without Mega Evolution) who would absolutely dominate SM Ash.
Paul's League team (which is a significantly weaker team than Paul's actual best team of six) would demolish SM Ash 6-3 at very worst, and that's accounting for Paul sacrificing Aggron+Gastrodon. That's generous given that if Paul led with Drapion and sent out Electivire second, he'd sweep SM Ash 6-1 (assuming SM Ash sent out his Pokemon from strongest-to-weakest). The only way SM Ash could put up a decent fight is if he was allowed to use a Z-Move with each of his Pokemon (for a total of 6 Z-Moves). In this case, Paul would still win pretty decisively despite giving SM Ash a huge handicap in his favour.
Also, Kukui is not a Champion. Even if he were in the anime, that title would be irrelevant given Kukui founded the Alola League and had no qualifications to it. Kukui's skill as a trainer was irrelevant when creating the Alola League, since he was a Pokemon Professor. You also forget that the games and anime are separate, otherwise Acerola wouldn't be getting beaten by Kiawe, let alone Kahilli losing in the Battle Royal like all the other fodder trainers. Hell, even in the games where Kukui was a more competent trainer, he lost to Lance who was an E4 member at that time.
Even in the games, Kukui was never Champion-level. In the anime? Kukui's Incineroar never demonstrated any feat that couldn't be easily replicated by even the weakest of Frontier Brains. You seem to think that Kukui must be particularly strong, when in truth Kukui in the anime is significantly weaker than Brandon (and gets steamrolled without a Z-Move), and would probably struggle against some of the lesser Frontier Brains too. It could just simply be that Kukui was vastly overrated, and that Gladion (and arguably SM Ash too) was already a better trainer than him.