Probably either Cilan,Misty,Brock or May.
Here is view on everyone:
Cilan:he had lot of battling experience as gym leader and by taking in account good use of strategy we saw he would be one of better battlers out there.His pokemon like Stunfisk,Dwebble or Pansage all showed impresive moveset and strength too(athough in his case every new move seems to be learned of screen taking away from credibillity).
May:didnt battled much in real trainer battles which could represent handicap for her in those kind of matches but she showed impressive skill and combination of techniques in contest battling rounds with her style as coordinator being similar to battling style Ash used in gyms with some things going in her favor.She also has quite a few good powerhouses like Blaziken,Venusaur or Glaceon with others not being too shabby either.Definitely among top ones judging by what she showed so far.
Brock:unfortunately while he has impressive collection of pokemon like Steelix,Crobat,Marshtamp etc his battling skills rusty over time barely battling after he left his gym leader position being mainly focused on taking care of pokemon being breeder.
He did showed some good moves like when battling May in contest round but we get to see with his lane performance vs inspector Joy recently that lack of frequent battling for long time left its mark.
Misty: is the only girl until Iris,which wasnt complete beginner when she started traveling with Ash,she is gym leader and her aim is to become water master trying to overcome their disadvantage preparing them for anything,during course of series having some impressive battles and wins like winning Princess tournament,Seaking contest,managed to top 8 in Whirl Cup(which has very strong competition),and some of her pokemon like Staryu are one of her strongest pokemon displaying strength vs Mollys Mantine(which was stronger than usual because of unown).She also has that powerhouse Gyarados,versatile Staryu and Corsola and well trained Politoad having a lot of battling experience.
She definitely shouldnt be underestimated being among better ones.
Dawn:showed impressive knowledge and use of her experience as coordinator and has some good pokemon like Quilava or Mamoswine.However problem is she doesnt have much experience in real pokemon battles and while same thing could be said for May in her advantage goes fact that she mimic Ash style when battling gym leaders with different kind of battling in contests giving her edge.
That and Dawn didnt showed outside from contests some impressive use of skill when battling against regular trainers like Brock at times,Misty or Cilan did.
Some combinations which are used in contests gaining points are great for such events,but wouldnt end up very effective in normal battles.
Iris:aside from competent and quite powerful Excadrill other members of her team like Emolga and Axew didnt showed much going for them for now and due to Iris not having so much battling experience like some other characters she isnt one of strongest Ash companions at moment.
Sure mole did beat 99 opponents but we dont know how good they were and this was local tournament with other characters like May,Misty etc winning them too entering tournaments with even higher competition.
Tracey
ossibly weakest battler out of all Ash friends.He almost never battled being watcher which explains his lack of interest for battling not having particularly good team.Scyther is his best bet but he is quite old being prone to tyre very fast.
So in my opinion strongest battler out of Ash friends is either May or Misty basing on what has been showed so far.If we count of screen battling experience and knowledge Cilan could fit in that group too but he needs to shows little more of his abilities to allow for complete judgment.
The problem with Misty is her team is all the same type, so she has no diversity in her pokemon. When you have all water types with almost all similar moves, there's not a lot she can do there. The other problem is her Starmie never used any Psychic attacks or any special attacks at all, so it just came across as a clone of Staryu.
Since when one type trainer was handicap???
Heck strongest trainers in world are one type trainers like E4 members,Loreli or Wallace use mostly water type and they certainly arent restrained because of it beig stronger than most trainers who use different types out there.
Yes Misty use mainly one type but at same time we are talking about most widespreaded and diverse type of pokemon in world being second in number of types its been paired up with having access to wider pole of moves being capable to use techniques from other types because of this aswell attacks specific for water pokemon starting to increase not bing only centered on water gun(as some like to believe).Being water trainer was Misty niche and one of reasons people found her goals and pokemon unique taking lot of skill and determination to become best that way.This made her different from others.
Speaking of Misty team using same techniques thats obviously not true either.There were several other techniques her water pokemon used wuith moveset over timer being enriched like bubble beam, Corsola using spike cannon,mirror coat and recover,Staryu using double edge or swift attack,her Goldeen used supersonic,Psyduck knew confusion and disable(with help of headache),Luvdisc ice beam,while Politoad(before Poliwhirl)used attacks like double team or swagger.
Some moments which comes to mind where she really shined showing her potential as trainer was in Whirl Cup against Harrison,Ash and Trinity,when she battled gym leader Dorian,when battling against Molly in third movie etc.
Way in which she interacted with water types earning their trust(referring to Gyarados)or when encouraging them to restore confidence back(referring to Poliwhirl)showed that her character has predispositions and potential which someone needs to make a name for itself.
Misty maybe didnt had luxury to have many battles like Ash or some other characters did but basing on what we saw she was certainly not some low class rookie being skilled as water trainer laving positive impressions on many other trainers(especially when battling in her own element water).
She is also along Iris only character which managed to win first real tournament early in series being nothing to scoof at.