Lets take a look at each character:
With Misty.She served role as Ashs coach and mentor pointing out his mistakes,giving him tips when it comes to choice of using pokemon in some battles aswell what kind of pokemon types are good against other.
She also provided moral support to him with her teasing remarks providing required push he needed to improve himself as trainer.
She also acted like rival during some occasions with both characters pursuing relatively similar dreams(of wanting to become best trainers out there)with Mistys portrayal of being better trainer than he is invoking in Ash anger and motivation to go forward(like when they battled for Totodile or in Whirl Cup for example).
Along with Brock Misty served as catalyst to help Ashs growth as trainer when he needed help the most during its rookie days with strong and deep friendship which was established between them leaving impact on his gradual change.
Brock-left pretty big impact on Ash character too.His help during pokemon battles,great deal of knowledge serving as coach and mentor helped Ash a lot being able to learn basucs in pokemon battkles,He akso quite ofzten provided help in battling astrategies along withngiving him tips o how to taoe care of his pokemon.
With May-it was more vice versa sort of case.In Hoenn Ash was already experienced and mature as trainer not being prone to making newbie mistakes allowing him to develop further his own style of battling without having need to rely so much anymore on others help or mentoring.This allowed him to take up mentor role toward May influencing her own contest style which later reflected during her performance in contests having more of a battling tone to it.
With Dawn-her practicing for contests,perfecting attacks for appeals etc provided to Ash lot of knowledge starting to take his battling strategy on whole new dimension using some of things he learned in his future battles which was showed to be proven success.They also shared several similarities being able to relate to each other.However most of it was based on moral and friendly support not invoking some particularly big change in Ash character imo.
From Max and Tracey-Ash barely learn anything new with no motivation or inspiration coming from their characters not leaving lot of impact on his character and battling style.Granted Tracey can be excused because of being in show for very short amount of time.
On Dento and Iris i will stay neutral since both of them were so far part of show for relatively short period of time with BW series not giving us full picture yet.
In nutshell characters which influenced Ash the most would be:Brock,Misty and to some notable extent Dawn.
Characters which influenced Ash character and his battling style the least would be:Max,Tracey and to certain extent May.
Squirtle learning hydropump).Or when he lost in Kanto league starting to selfpity himself being able to get out of depression thanks to Mistys encouraging and motivating speech.
On top of that she often mentored him along with Brock about type weakness and what kind of approach he should tale toward certain pokemon with certain rivalry which was present between them invoking in Ash desire to try harder which later reflected on his battling style.