Funnily enough its mix of Kanto,Johto and Hoenn Ash for me.I liked his starting days when he was complete rookie being bratty,hotheaded and naive as hell.He made mistakes,didnt want to accept advice from Misty or Brock being always full of himself and sure in abilities just to backfire on him later.His rivalry and clashes with Gary were also always fun to watch bringing drama and tension to show.
In Johto you could tell he didnt changed much in personality department,but some small signs of starting to mature were present with Ash becoming less stubborn,more calculated and definitely more experienced as trainer compared to Kanto days using more of strategy compared to tactic"with head through wall and risk as much as you can"which was often recipe for his battles back in day.You could tell that companions moral support,tips in battles,Brock wise suggestions and motivation from Misty side with there existing friendly competitive fire between them, helped Ash to grow as trainer and person leaving mark on character.
What i liked about him in Hoenn was progressive sign of maturity character went through becoming more levelheaded,calm and self-critic wanting to learn on mistakes while keeping his bratty and funny aspect of character.He became "grown up enough"to take mentor role on someone else(in that case being May)helping her when training and preparing for contests which portrayed Ash in different yet interesting and unique light.
In Sinnoh he has matured more going forward compared to Hoenn days but i find that as positive thing.This really reflected on his skill as trainer and experience growing in worthy and respectable trainer having great relationship with Paul with their rivalry being one of most substantial and well developed story arcs this show produced.His friendship with Dawn and moral support they provided for each other along with Ash being wiling to think "outside the box" starting to mix contest techniques he saw from Dawn in his methods of training and battling strategy resulted with great success.
His steadiness being adamant about his way of training pokemon being angered and frustrated with Paul harsh and sometimes cruel methods,passionate side fighting for his beliefs with lot of care being pout in abandoned Chimchar and training which beard fruits on table evolving in formidable Infernape depicted Ash on whole new level making me to care for his growth and all loses and bad times he went through.
He was still funny ,sometimes naive and bratty which can be noticed in clashes he had with Paul and Dawn,he was still clueless and easy going about several things(like when he fell asleep on classes)causing annoyance and discredit from others(Like Angie),but he was also more collected,strategic and open minded toward new ideas evolving as character.
And i liked that about him.