None of them really gave me a hard time...
I guess the most difficult for me was Lt Surge, because at this part of the game I didn't have strong Pokémon against electrik types yet.
Either Brock or Blaine. Neither of which I enjoy.
Brock because I'm a sucker for Charmander and Blaine because I'm too lazy to do extra training and every time I play the game he manages to OHKO my water/ice type