Most definitely Sinnoh Ash. He displayed a surprising level of maturity not seen previously...or after, unfortunately. He actively learned from his mistakes, actually developed a battle style that didn't involve just picking the bad type matchup and winning because plot armour, wasn't afraid to try new things, incorporated elements of Dawn's style into his own, and actually worked with her to develop her style, too. He's never had that kind of rapport with his travelling companions before or since; he generally prefers to do his own thing and let them do theirs, so seeing him getting actively involved and sharing his experience, and being willing to learn himself, was wonderfully refreshing, and it actually made him something he hadn't been for a long time: likeable.
Yes, I know Ash took part in a contest in Hoenn, but it was literally just so he could have a battle with May in the final...Ash, who had no prior contest experience, got to the final. And managed to draw with someone who had been doing contests for the entire show. Compare this to his Sinnoh Contest, where he entered with Buizel in an effort to better understand its strengths, and got knocked out fairly early on. It was a much more natural, better-written scenario.
He also had the best foil he's ever had in Paul. There was real enmity between Ash and Paul, and it highlighted Ash's character strengths and weaknesses and, more importantly, pushed him to overcome the latter. None of Ash's other rivals have ever really done that for him; they're generally rivals because they're trying to achieve the same goals. Paul was Ash's polar opposite in every way, and initially he was the better trainer too, because Ash's emotional immaturity led to him repeatedly losing. So his league battle against Paul, winning with a Pokemon Paul himself had previously discarded as useless, was one of the best and most meaningful victories he's ever had.
Even though he has finished higher in leagues, I would even go so far as to say he was the most successful Ash to date. Tobias was put deliberately in his path to make him lose, but Ash took out his Darkrai AND his Latios after that. TWO Legendaries. That was far and away his most powerful rival and, whilst he didn't beat him, just taking out that Darkrai was a monumental achievement for him, never mind taking out another Legendary after that. I think we all know that the only reason Ash lost to Alain is because of bad writing, and the only reason he won Alola is because he didn't have any serious competition, and his team was ridiculously overpowered. Ash didn't have a singular powerhouse in his Sinnoh lineup, which made his battles more enjoyable and his victories much more satisfying.
As a general note, I think aesthetically he looks more mature as well, with Kalos Ash being a close second. Unova Ash looked like he dropped three years or so.
