No, Battle Frontier or DP would have been. XY doesn't count since it came after BW destroyed everything including natural continuity to past series and Ash's progression as a trainer and character.
DP would've been the best place to do it (if they hadn't done it by Johto).
Felt like the culmination of OS and AG. He brought most of his reserves back. DP had the best battle writing and strategies. Perfect time to elevate Ash into champion trainer status then.
BW was meant to be a reboot, they should've committed fully and not used Ash.
And here's something I realized in hindsight. These potential endings? They aim too small and simply cut Ash short.
Yeah, Johto had Ash beat Gary and the original trio splitting up for good. But we already saw Elite Four Members like Bruno and Lance, trainers more powerful than Ash. Ash and Gary improved since their last League, but they have a personal rivalry to deal with, no where near the top of the top. Ash is then knocked out by Harrison with Gen III Pokémon, indicating a new region to explore and new Pokémon to catch. In short, while Johto is the end of Ash's journey as an upcoming, inexperienced trainer, he wasn't anywhere close to being a Pokémon Master. Just a decent trainer who is now better than his rival.
The Advanced Generation did gave Ash a chance to prove his worth through the Battle Frontier, which had him facing very powerful elite trainers and Legendary Pokémon. He even got his old Kanto trio team back, which seemed appropriate given the 10th anniversary approaching. That would have been the right ending... but is Ash strong enough to defeat the Elite Four now? Are the Frontier Brains really on par with the Elite Four? Not to mention about the Pokémon League, which Ash hasn't reached beyond Top 8. There's a lot of things for Ash to prove himself. A lot of unfinished things.
Diamond and Pearl seemed to offer a great conclusion for Ash... except not really. Ash's Sinnoh League was cut short. Sure, he advanced to Top 4, but it was clear the climax of the story was with a personal rival in the form of Paul, and then they had to boot him out of the tournament with Deus Ex Machina Tobias. There was more promised to us with DP, namely a Champion League where Ash could face Cynthia. Yet, it seemed clear that Cynthia was way beyond Ash's current status. And how do we establish that Cynthia is not only the Champion of Sinnoh, but a Champion that beats other Champions.
BW, the less said, the better. XY could have been a great ending for Ash... if it weren't for the fact that Team Flare arc took much of the climax, ensuring that Ash won't have another chance to face Diantha or the Kalos Elite Four. Not that it matters considering the actual finals. SM did end with Ash's victory and Ash facing Kukui... but it always felt like Ash could go to the next region/generation because of how new the Alola League was and how lower the stakes were.
What these series endings have in common is that while they are personal achievements that Ash has made for himself, they were always relatively small, overshadowed by bigger challenges yet to be faced. As long as the Elite Four and the Champions remain unchallenged, there would always been a reason to follow Ash's journey.
What Journeys did was put a cap on the hierarchy by establishing Leon as the world's greatest and strongest trainer. Not simply an undefeatable Champion of Galar, plainly an unbeatable Champion of the World. He's introduced crushing Lance in their match and this established a confirmed ranking among the Champions, with Cynthia being the second strongest. And with Ash wishing to face Leon and beat him, suddenly there was no ambiguity. If Ash defeats Leon, he will be at the top of the world, the closest he'll ever get to being a Pokémon Master. And if there is no one above him, then there is very little reason to keep following Ash since Title Defense would change the very formula of the Pokémon anime.