Erikas the easiest for me since I chose Charmander. But if you choose Squirtle or Bulbasaur then probably Brock or Giovanni are the easiest. Misty is also pretty easy for Bulbasaur-choosers. Squirtles kill Blaine and Charmanders burn Koga. Lt. Surge is dangerous generally but especially to Squirtle-ppls. Honestly its all about whatever starter you choose and the type of pokemon you train. However, leaders like Sabrina are pretty well-rounded I guess..