I love Pokémon and have been watching the anime/cartoon ever since I was a little child! Pokémon is still popular!
Battle Against Lt. Surge - Pikachu vs. Raichu
This is one of the very first serious opponents that Ash Ketchum fought against. Lt. Surge had very well trained and powerful Raichu at the time. Pikachu battled Raichu, which ended in Pikachu taking some very serious damage. Being Pikachu's evolved form, Raichu won the battle with very little effort. Pikachu actually landed in the Pokémon center because of his injuries. Then, Ash received a thunder stone from nurse Joy but Pikachu decided to not evolve. With Brock's help, Ash focused on boosting Pikachu's speed to defeat Lt. Surge and his Raichu. This battle showed off Pikachu's will power and strength early on. As time went on, Ash's Pikachu is clearly one of the strongest Pokémon in the series. This epic battle brought back so many childhood memories for long term Pokémon fans.
Battle Against Blaine for the Volcano Badge - Charizard vs. Magmar
Charizard is one of Ash's first Pokémon to fully evolve. Most Pokémon he caught early on in the series almost never evolved. But Charizard is an exception. From the obedient and cheerful Charmander to the powerful and arrogant Charizard. Upon his evolution, Charizard stopped obeying Ash because he perceived his trainer as weak. It will only battle strong opponents at that time. Blaine showed up with his powerful Magmar and ignited Charizard's battling spirit. This led to an amazing fire type battle that resulted in the volcano badge for Ash Ketchum. This is one of the first battles that Charizard showed off his formidable power.
Battle Against Gary Oak - Johto League Silver Conference
Gary Oak, grandson of the famous professor Oak, is one of Ash's first and most memorable rivals. From the very start, Gary was always a step ahead of our hero. Ash never seemed to catch a lot of powerful Pokémon. He trained whatever that came his way. On the other hand, Gary caught powerful Pokémon and evolved them. Looking at his data on Bulbapedia, Gary had Blastoise, Umbreon, Electivire, Nidoking, Arcanine, Scizor, Golem, Alakazam, Pinsir, Kingdra, and more at the time of Silver Conference. All of these Pokémon pack a serious punch compared to some of Ash's Pokémon at the time. Nevertheless, after a very fierce battle, Ash came out on top unexpectedly. It was another one of Charizard's victories because he took out 3 of Gary's Pokémon in the full 6 on 6 battle.
Battle Against Noland - Charizard vs. Articuno
Another one of Charizard's victories is the fierce battle against a legendary Pokémon, Articuno. Back in the early days, Ash relied heavily on Charizard for his most important battles. Noland was very strong at the time and had Articuno on his side. Nevertheless, Charizard emerged victorious through a very difficult battle.
Battle Against Paul - Sinnoh League Championship
Besides Gary Oak, Paul is another very memorable rival that Ash fought against in the Sinnoh region. Being very different from Ash, Paul exclusively relied on power. The way that Paul treated his Pokémon was also very annoying to many. However, his Pokémon fought fearlessly against all opponents and appeared visibly upset at Paul's lost. Even though nobody liked Paul, he is still one of the best rivals that Ash had throughout his travels. After Paul's defeat at Ash's hand, he is now a lot more considerate of his Pokémon's feelings. The epic Sinnoh League battle between these two is a must watch!
Legendary vs. Legendary - Hoopa the Clash of Ages
The battle between powerful legendary Pokémon is full of amazing clashes. Hoopa, the mischievous psychic / ghost type mythical Pokémon, summoned many legendary Pokémon and forced them to battle each other. No battle in the series featured so many powerful pocket monsters. This battle is truly one of kind. In one scene, Ash battled alongside a mega Rayquaza.
Battle Against Sawyer - Kalos League
Don't underestimate the pint sized trainer Sawyer. He packs a serious punch. With Pokémon like Salamence, mega Sceptile, and Aegislash, Sawyer made it very challenging for Ash. Even though Sawyer was not a significant character in the Kalos region, the sheer sight of Salamence, mega Sceptile, and Aegislash in action is nothing short of a fierce 6 on 6 full battle. Not to mention, Ash had to use his ace Ash-Greninja to finish off the powerful mega Sceptile.
Battle Against Alain - Kalos League
Alain has mega Charizard x, enough said. This adult Pokémon trainer is one of the most powerful in all of Kalos region. He and his trusted partner Charizard finished off 10 mega evolved Pokémon in quick succession. Talk about sheer power. It is not surprising that his battle with Ash is one of the most action packed. Of course, Ash was unable to defeat Alain in the Kalos League.
Battle Against Kukui - Pikachu vs. Tapu Koko
This is the battle that crowned Ash Ketchum a champion. Over the past 20 years, our boy has been diligently working his way from novice trainer to Pokémon master. Ash was finally able to accomplish his goal at the end of Sun and Moon. With his strong team and faithful partner Pikachu, He was able to defeat professor Kukui and claim the title of champion of Hawaii (I mean Alola!).