I used to have quite a bit of difficulty with Sabrina - I'm not sure why though, she wasn't that difficult. I guess when I was younger (and played FRLG) I didn't really care too much about type advantage - raw power was what mattered to me. Now that I'm a little bit older, and that much wiser, I guess I would teach my Blastoise bite/crunch. I never used to teach it that because I thought "I water Pokémon shouldn't learn a DARK TYPE move, that's reuniclus ridiculous".
Another Gym leader that I had a mild difficulty with was Lt. Surge. I hated Diglets, so I never used them against him. I had no ground types and I always lost (except for that one time I had Geodude) However, now I always teach something dig, so that I can overcome him.