Its a tie between May and Misty for me.
May: because she showed to be rather strong trainer having quite impressive collection of pokemon. When you take a look at her Venusaur, Blaziken who has been longest lived and most experienced member on her team, Wartortle or gracious Glaceon you can say she has diverse and well balanced team. Granted she didn't battled much in real trainer battles which could represent handicap for her in those kind of matches, but she showed impressive skill and innovative approach in combing techniques to bring surprise over opponent(such as Psychic power up or focus ball) in contest battling rounds, could could bring her an edge over other opponents.
Misty: because as trainer she displayed wide amount of knowledge being skilled which knows how to turn unfavorable position in her favor and establish strong bond with pokemon. As she showed through battles(one which comes to mind was against Molly, having Staryu counter whirlpool and bubble beam with rapid spin in different direction, being on equal ground with powered up by Unown pokemon).
She also had several notable achievements doing well in big tournaments she entered, being one of few which weren't rookies when meeting Ash beating him.When taking in account fierce Gyarados, versatile Corsola or Staryu, strong Politoad, full of potential Starmie or Psyduck etc you know she has powerful team which can give hard time to any opponent.
Not to mention she probably trains and battle a lot as gym leader giving her lot of battling experience.
Granted as mono trainer this would put her at disadvantage against more diverse teams, but in nutshell it doesn't matter if you have evolved or un evolved pokemon, nor how many different types you have. What counts is how skilled trainer is, how strong his pokemon are and if they posses lot of battling spirit.
E4 members all use one type and they are among best trainers in world, most gym leaders use one type, even some champions like Wallace. So its not like mono trainer cannot be formidable force.
I'd have to say May for reasons already started. Plus, she had the most travel time and we got to see her grow and develop the most in two different regions whilst Misty really didn't improve in the time she traveled with Ash and Brock.
Misty definitely improved during her travels to become water pokemon master (although that dream was left unfulfilled). In Johto her skills became a lot better compered to Kanto days, she achieved several things like coming top 8 in Whirl Cup, winning Seaking championship, Alto Mare race etc with several trainers like gym leader Dorian, Andreas, Trinity etc recognizing her battling abilities.