The Gym leader you mostly struggled?

Liza and Tate, every single time. I dread battling them because I've never got anything to resist Psychic attacks and I just generally hate double battles.

And there's also Claire because her Kingdra is a pain to take down with neutral attacks.

Sabrina was a problem in R/B for me but oddly enough not in Yellow. Koga was the problem in Yellow.

Brock in pokemon yellow. I still remember I had to train my butterfree until lvl 15 just to beat his onix. Pikachu was totally useless during the 1st gym in the 1st gen.