Giovanni was hard for me as a kid with Yellow. Of course, I was a dumb kid so I was unprepared, but Fissure. All the Fissure. Then there was Persian's Slash, and of course this being Gen I, Slash was always critical. And then Rhydon was just awful. I remember wandering around in the backyard telling myself not to go inside until I beat him because I was crying so much and didn't want anyone to see me.
But hands down, Whitney was the worst no matter what I did. The fact they kept her hard in the remake is what freaks me out so much, like I thought I was prepared to take her down, but it still took me like a good fifteen/twenty minutes to finish the fight, and I was playing it before class started, too. All that pressure.
I honestly forgot Tate & Liza were hard if you weren't prepared for them. Their Pokémon having Levitate I think is what had thrown me off the first time around, next to their Psychic/Rock dual-typing, they just resisted Water, Grass, Dark, and Fighting attacks--just about everything except maybe Steel? Took me a few times to finally beat them. Dick move, Game Freak. Really no wonder Ash had to pull Thunder Armor out of his ass to beat them.