The most significant character development happens series to series. From a macro perspective, it is obvious that Ash grew as both a person and as a trainer from OS to DP. It wasn't an entirely smooth transition, and often much of the transition happened at the start of each series, where the writers decided from the get-go that this version of Ash was going to be more mellow, mature, and skillful as a trainer.
However, stuff like Ash no longer taking badge handouts, his rise in maturity throughout Charizard's arc, his change in demeanor after losing in the leagues, and looking past the crushing defeat at Lake Acuity all point to ongoing growth. I actually don't think there's tremendous change within a series, but more so his character development should be viewed holistically.
Again though, this is more fitting of OS-DP.
BW entirely 'reset' Ash as we all know. XY made him 'mature', but I don't think he was any more mature than he was in DP. It felt like a DP Ash if he didn't have a rival like Paul to challenge him, or companions like Misty/May/Dawn/Iris with whom he could banter.
SM Ash turned him into more so of a character concept that can be shifted to whatever series tone, which in SM's instance, was tropical light-hearted slice-of-life and humor. Yet, he still retained a lot of his skill and experience. Lastly, JN is this smorgasbord of different Ashs.
At the end of the day however, nobody looks at the Pokemon anime as a paragon of writing and character development. It's more or less a big advertisement for the games and toys. Half the episodes in this series are formulaic COTD/POTD episodes versus Team Rocket. And Ash's journey should have ended by a post-DP finale arc.