I don't remember struggling with any particular gym leaders but I guess the hardest for me personally was Whitney honestly. On my first battle with her, I remember sending out my starter, Quilava up against her Miltank. At the time, I had handled Clefairy easily so I thought this battle would be a breeze. Well, I was wrong. Completely wrong. Incorrect.
After flame wheeling it endlessly,I eventually managed to get Miltank into it's low HP range. I decided this wasn't so bad after all and that maybe victory wasn't as far away as I excepted. At this point my heart was pounding, I pressed the move that would finish it off. But what happens? A healing item. I remember the moment vividly, my mouth twitching and me ready to scream. It had taken me the past, who knows how long to whittle down it's health only to find my efforts have died in vain. Another fifteen minutes later, Miltank was back in low HP.
I smiled to myself, usually Gym Leaders only used a single potion throughout the gym battle. "Prepare to go down you dratt-" then I paused as Miltank used Milk Drink and recovered its health. I remember going red and then after another ten or so minutes of continuous healing and almost getting knocked out by powerful Rollouts and Stomps, somehow victory managed to still find its way. I guess the saying that there's always light at the end of the tunnel is true after all.