The Kanto region is where the Pokemon journey began, and it played host to some amazing Gym Leaders and battles.
In the games and the anime series, none of the Kanto Gym Leaders were pushovers. In fact, it took a long time for Ash to actually gain a badge by winning a first battle, rather than a rematch or just being given the badge.
Whether in the anime or the games, there are a few Gym Leaders who could've benefited from having a different Pokemon on their team.
Note: This article is subjective and reflects the opinion of the writer.
3 Pokemon that could replace Kanto Gym Leaders' Pokemon
#3 - Psyduck
Everyone knows that Misty and Ash did not get to finish their Gym Battle. This is just one of many examples in Pokemon where Ash receives a Gym Badge without technically earning it. In the show and original games, Misty has a Staryu and Starmie to battle with.
If Ash and Misty were able to have a fair Gym Battle, Psyduck should have replaced her Staryu. While a bit unhinged, Psyduck has shown its immense power time and time again. It would have been a risk, but it could have wiped the floor with Ash's team.
#2 - Electabuzz
Lt. Surge's Raichu absolutely decimated Ash's Pikachu the first time they battled. In the Generation II games, when the player travelled to Kanto, Lt. Surge did have an Electabuzz on his team. It would have been a helpful Pokemon far earlier than that.
If Lt. Surge had an Electabuzz to battle Ash or the player in Red, Blue, or Yellow, things could have turned out much differently. Electabuzz is a very intimidating Pokemon that, honestly, would have scared Ash and Pikachu half to death in the beginnings of their adventure together.
#1 - Charizard
If Blaine's Magmar was replaced with a Charizard, fans would have gone ballistic. The battle between Magmar and Ash's Charizard was arguably the best one in the entire Pokemon franchise up to that point. The Magmar was found on his team in Generation II games, but not in the first Generation of games.
Regardless of that, this is in terms of the anime series. If Blaine put a Charizard into battle as his last Pokemon against Ash, who knows what the outcome could have been. One thing is for sure though, a Charizard versus Charizard battle in the Cinnabar Gym would have been as intense as it gets.