Everyone who has ever played the games or watched Pokémon has a few Gym Leaders that stand apart as particularly memorable. Whether it's due to the Pokémon they chose, their appearance or a memorable encounter, a few gym leaders stand out from the rest.
Of all the gym leaders, the Kanto region has some of the most popular and memorable. This includes those who appeared later in the franchise to replace gym leaders who left Kanto as of Pokémon Gold and Silver, as they are as much Kanto leaders as those they replaced.
Janine
Janine replaced her father, Koga, in Pokémon Gold and Silver when he advanced to join the Elite Four. As the new Fuschia City Gym Leader, Janine felt like a slightly inferior counterpart to her father. Sure, this poison gym leader remains a challenge to anyone who faces her, but her only memorable role outside the anime is in the Pokémon Adventures manga, where she reveals she feels directionless without her father.
Koga
Koga is easily the least interesting of the original Kanto Gym Leaders. Koga is a poison ninja who is promoted into a member of the Elite Four but has the least interesting confrontation with Ash in the anime. While his membership in the Elite Four adds needed variety to his roster, his team consists of multiple repeats (Koffing in Red/Blue and Venonat in Yellow). To make matters worse, his gym consists of invisible walls, making it the most frustrating to traverse.
Erika
Erika is another less-than-memorable Gym Leader, both in the anime and the game. While players with Charmander probably had an easy job plowing through her team, most others struggled against her Vileplume or Gloom. Her team in Pokémon Gold and Silver is arguably more dangerous, though her gym is relatively easy. Even worse, her confrontation with Ash in the anime is really only memorable due to Team Rocket convincing Ash to sneak into her gym dressed as a girl to face her.
Lt. Surge
Neither Surge's backstory as a war veteran nor the confusing trashbin-related minigame to unlock Surge at his gym are particularly good reasons to remember him. What stands out about Lt. Surge is he's the first gym leader to not only beat Ash but thoroughly break him and Pikachu in combat. The anime fight in Vermillion City ranks as one of the most memorable moments in the original anime. To make matters more interesting, Surge's manga counterpart is a particularly devious adversary.
Brock
Depending on who you choose as your starter Pokémon, Brock can be either an uphill challenge or a breeze. While Brock's Pokémon Gold and Silver roster is far more dynamic, his original team is relatively weak. Obviously, though, Brock stands out due to his role in both the anime and manga. In Pokémon Adventures, he's a powerful ally in the fight against Team Rocket. In the anime, he's Ash's companion and mentor, traveling alongside him for five regions. However, Brock fares much worse as a gym leader, given he quickly abandons his post after meeting Ash.
Giovanni
Giovanni is the head of Team Rocket and the secret gym leader of Viridian City. Giovanni stands out as one of Pokémon's best bad guys and most notable gym leaders. Though Giovanni never faces Ash as a gym leader, he beats Gary by bringing out Mewtwo. In the games, Giovanni is a surprisingly disappointing eighth gym leader. With primarily Ground Pokémon, anyone who has a water or grass type -- two of the three available starter types -- can easily beat Giovanni.
Blaine
Blaine is the fire gym leader of Cinnabar Island. His initial fight is challenging but not particularly overwhelming. However, Blaine really picks up the slack in later appearances. After his gym is crushed in a volcanic eruption, Blaine responds by holding a secret, secondary gym, showing that even nature cannot stop him. In the anime, his fights with Ash span multiple episodes, with the Charizard and Magmar fight ranking as a highlight in the early anime. However, Pokémon Adventures gives Blaine an edge over other gym leaders by presenting him as Mewtwo's creator.
Blue
Blue becoming Viridian City's gym leader following Giovanni's departure acts as the culmination of their arc. He starts as a cocky rival to Ash before finding the fault in his own hubris and settling down. There's a poetic sense of maturity about Blue as a gym leader in Pokémon Gold and Silver. It also doesn't hurt that Blue is a particularly powerful trainer, with a particularly diverse team that can actually challenge players by the time they face him in-game.
Misty
Misty is one of the first gym leaders players encounter but is more famous for becoming Ash's traveling companion in the anime. However, as a gym leader, Misty is actually pretty challenging. Her Pokémon team in Pokémon FireRed and LeafGreen is amped up, with Starmie's new moves like Recover making her hard to overcome and her Pokémon Gold and Silver roster presenting players with a real challenge. Unlike Brock, Misty returns to her gym, becoming an unstoppable challenge to all fledgling trainers with an all-powerful Gyarados.
Sabrina
Of all the Kanto gym leaders, few are as downright terrifying as Sabrina. Her team was virtually unstoppable, as Psychic Pokémon had no weaknesses, had advantages over fighting and ghost Pokémon and could recover from damage. The manga makes her a Team Rocket executive who manages to fuse the Legendary Birds. In the anime, she turns Misty and Brock into dolls before psychically tormenting Ash with a virtually undefeatable Kadabra. Sabrina is easily the most terrifying threat anyone in Pokémon has ever faced.
