Easiest: Erika or Koga, Both of them are pathetic with Charizard to be honest, and I always had a Psychic type at the time to take care of Koga.
Hardest: Misty...ohhhhh Misty... It's easy with Bulbasaur, but man is it crap with Charmander. If you didn't get a Pikachu, you were screwed. But if you did, the hardest boss would have to be the final one.