In Pokémon, few moments are as thrilling as facing the gym leaders. Players have enjoyed and appreciated these face-offs since the early days of Kanto all the way into the more modern sports arenas of Galar. While the exact nature of these battles has changed somewhat in recent years, the original gym leaders have always been considered a cut above the rest, especially in the anime.
In Pokémon: Indigo League, the gym leaders were more than just a challenge. The Kanto leaders and the battles Ash had with them were essentially a test of will, with the gyms representing a seemingly impossible challenge for Ash to overcome. This was in no small part due to the actual physical dangers the gyms posed to Ash and his Pokémon. While some were much laxer than others, a few were completely unfair to ask of a 10-year-old boy.
8) Cerulean City
Ash's battle at the Cerulean City Gym was arguably his easiest win, as well as the easiest gym in the region. In fact, he didn't even have to battle for the gym badge. Run by Misty's three older sisters, the gym had been essentially handing badges out to trainers due to the sisters not enjoying Pokémon battles. If it weren't for protests from both him and Misty, Ash could have easily walked out with a badge without even breaking a sweat.
Ultimately, Misty decided to face Ash in her sisters' place and give him a proper battle. Unfortunately for Misty, Ash was able to take the lead in their two-on-two battle even without Pikachu (who was uncomfortable fighting against a friend). While Team Rocket interrupts their match, Ash is still awarded the gym badge because he was winning before the interruption, and would have definitely won if Pikachu participated.
7) Celadon City
This gym battle is a little hard to gauge due to the fact that Ash was almost definitely losing. After sending out both his Bulbasaur and even Charmander and having them both defeated, the likelihood of him winning was practically null. He might have had Pikachu ready to fight, but it's hard to imagine even him being able to beat Erika's Tangela and Gloom on his own.
The only reason Ash got this badge is because Erika was grateful to Ash for saving Gloom from Team Rocket after they interrupted their battle. While it would be easy to put this battle higher on the list due to Ash losing most of the time, the only challenge was that Erika was simply the better trainer; there was no danger outside of the norm.
6) Fuchsia City
Ash's battle against Koga was surprisingly straightforward compared to some of the other Indigo League gyms. Even though he was going up against a ninja, Ash didn't have to worry about any kind of gimmick. All Koga really seemed to want was a normal battle with Ash. The only real condition was that Koga demanded he and Ash battle before the group would be allowed to leave.
Arguably, the most dangerous part of the battle was when Team Rocket showed up -- and considering they were all beaten by Misty's Psyduck, it's pretty easy to see that there was no real danger.
5) Vermillion City
In Ash's battle against the hilariously large Lt. Surge, the danger posed was more philosophical than anything else. The first time Ash faced Surge, Pikachu was easily defeated by the latter's Raichu, the evolved form of Pikachu. Surge mocks Ash for not evolving his Pikachu, implying he is a weak-willed trainer with only weak Pokémon.
Ash struggles with the idea of what it takes to win, but ultimately chooses to let Pikachu make the choice. As every fan knows, Pikachu doesn't evolve, deciding to use his superior speed to ensure victory after realizing that Surge's Raichu evolved before that specific stat could be properly developed. While this gym didn't really offer the kind of dangerous aspect as those further down this list, it was one of the first times Ash was forced to question himself as a trainer and had to regain the confidence to keep fighting.
4) Pewter City
Probably one of the most important and beloved gym battles in the entire franchise, this was the first time Ash was truly tested as a trainer. Fighting against Brock with only a Pidgeotto, a Butterfree and a Pikachu, Ash was at a huge disadvantage from the start. Not only were Brock's Pokémon stronger, but they had type advantages all around, resulting in Ash soundly losing the first time they battled.
It was only after Brock's father helped supercharge Pikachu that he stood a chance, but even then, Ash won by sheer luck. As Pikachu and Brock's Onix were fighting, the sprinkler system was set off, and the falling water resulted in Onix's defense and resistance to electricity dropping dramatically. Much like the Vermillion Gym, Ash's battle against Brock was more of a test of will than anything else. The reason it ranks so high is that it was one of the few times Ash was close to possibly giving up his dream for good. The victory was important because it helped Ash gain confidence as a trainer.
3) Cinnabar Island
One of the strangest gym battles of the first series, Ash's fight with Blaine was definitely something to remember. Forced to battle on an active Volcano, the 7th gym really raised the stakes. Due to the conditions, it was hard for Ash to figure out how to actually beat Blaine with the Pokémon he had. While he had a water-type Pokémon in Squirtle, the rest of his team wasn't well-suited to the challenge aside from his disobedient Charizard.
This gym ranks as high as it does mainly due to how Ash felt the need to forfeit his first challenge after seeing how much danger Pikachu was in. This was one of those Pokémon moments where Charizard was the key to victory -- a recurring plot device throughout much of the series. Being a fire-type Pokémon with the ability to fly, Charizard only needed to concern himself with his opponent, making the stage's environment as a whole quite inconsequential.
2) Viridian City
The eighth and final gym bout Ash had to face was one of the most dangerous yet ultimately disappointing battles in the series. Rather than face Giovanni, the true leader of the gym and head of Team Rocket, Ash instead went up against Jesse, James and Meowth. The gimmick of this match was that Ash and Jesse felt the pain of their Pokémon when they were hurt, making the battle quite dangerous.
Considering they were using multiple Pokémon at once, there was a truly high risk to Ash. Thankfully, he managed to pull a win, but unlike some of his victories, he truly earned this one.
1) Saffron City
Ash's fight against Sabrina was by far the most physically dangerous of the first series. Sabrina's use of psychic powers and Pokémon already made her a tough component, but the true danger lay in her warped mind. Obsessed with her own psychic powers, she developed two separate personalities -- one that was cold and power-driven, the other a little girl who just wanted to have fun.
Ash's group was literally shrunk and almost crushed at one point, and at another, Brock and Misty were transformed into dolls that had full awareness of their situation, but without the means of doing anything to change it. At no point in the series did Ash ever encounter a human being with this level of power, and if it weren't for Ash's recently acquired Haunter making her laugh, she would have surely captured Ash and done what she did to Brock, Misty and her own mother -- or quite possibly something even worse.