I haven't played Gold, Silver, or Crystal because the internal battery in my Silver game dies out repetitively, so it seems as if I have no say in this thread about the Johto Region (however I am playing through ShinyGold). It seems as if the majority agrees that Whitney of Goldenrod City is the most annoying gym leader, and I am working through Falkner as of now and I have yet to experience Whitney's battle style. In the games and regions that I have played through, I will let my opinion be known. In Kanto of the games FireRed and LeafGreen, I really thought all of the gym leaders were pretty easy to beat. If I had to choose, it would have to be Koga of Fuschia City or Giovanni of Viridian City. Koga's poison inducing status tactics can wear down your Pokemon incredibly fast, making your Pokemon extremely easy to beat. His poison pokemon were also very bulky and annoying. Giovanni was pretty strong in terms of power. His pokemon did a huge amount of damage to my team since most of the time my team was extremely weak to ground type attacks. In Hoenn of the games Ruby, Sapphire, and Emerald, I had a strong opinion that Norman of Petalburg City and Winona of Fortree city. Slaking was very bulky, not to mention he had two of them. I didn't use the Truant Protect combination to beat Norman, so Slaking kept dealing massive amounts of damage to my team. Winona's Altaria really ticked me off too. Dragon Dance from Altaria helped it do massive damage to my team, and after the speed boost, none of my very fastest Pokemon could outspeed Altaria. Dragon Dance gave Altaria a huge poost in it's STAB move Dragon Claw. 120 base power empowered by Dragon Dance's 1.5 boost each time. Altaria was bulky too so even if I did manage to survive a Dragon Claw, I couldn't get a really good hit in Altaria. Full Restores annoy me all the time. In Sinnoh of the games Diamond and Pearl, I am agreeing with the majority and say that Fantina was the most annoying gym leader. Why? Mismagius was incredibly hard to take down, but she was mostly annoying because of Drifblim. Drifblim had Aftermath, and I had barely any special attacks on my team. At common points I forget that Drifblim actually carried Aftermath, and that had costed me my strongest Pokemon. Drifblim was extremely bulky, too. Fantina used Full Restore on Drifblim, which caused me to throw my DS onto the ground. Drifblim's large HP stat was hard to take down, and I had to take it down twice thanks to full restore. In short, my candidates are...
Koga of Fuschia City, specializing in Poison types. (status, bulky opponents)
Giovanni of Viridian City, specializing in ground types. (hard-hitting opponents)
Norman of Petalburg City, specializing in Normal types. (bulky, hard hitting opponents)
Winona of Fortree city, specializing in Flying types. (bulky, hard-hitting opponents)
Fantina of Veilstone City, specializing in Ghost types. (bulky stall Pokemon, status)