The fourth generation of the anime was Pokemon at its very best. Ash was at the top of his game during his travels through the Sinnoh Region. Forced to reinvent his entire approach to being a trainer, he was a completely different person by the time he reached the Sinnoh League Championships.
Ash's history of coddling his Pokemon and releasing the stronger ones has made it difficult to take him seriously as a trainer. During his Sinnoh Region campaign, he was able to push his Pokemon harder in combat without compromising his core values that they are friends first and battlers second. Unfortunately, the show wasn't ready for him to win a championship yet so they sic'd some Legendary Pokemon on him to avoid the proper conclusion to his greatest journey.
This improvement as a trainer is largely thanks to Ash's rivalry with Paul. As the complete antithesis to Ash and everything he stands for, Paul represents the biggest obstacle Ash has ever had to overcome. It's bad enough for him to see how abusive Paul can be toward his Pokemon, but his rival consistently, and decisively, winning their battles throughout is salt in the wound.
Though Ash never wavered in his approach of loving his Pokemon, his futility in battle against Paul finally forced him to accept that affection is not enough. If he wants to be a great trainer, he has make his Pokemon work harder without venturing too close to his rival's methods. This balancing act brings out the best of both Ash and his Pokemon as his passion for battle rubs off on them.
Ash's Sinnoh Pokemon have the most personality of any of his Pokemon to date. His Turtwig is selfless and a peacekeeper, while his Gliscor grows from a crying Gligar that can't fly into one of his strongest and most reliable Pokemon. He trades his Aipom to Dawn when it becomes clear it prefers contests, showing it doesn't have to be a bad thing when a Pokemon and trainer have different goals. In return, he gets Dawn's battle-hungry Buizel, which is basically Ash in Pokemon form.
Infernape has the most compelling character arc ever given to a Pokemon in the entire franchise. Starting off in Ash's possession as a timid Chimchar, it was so abused by Paul that it was brought to tears the first time its new trainer's other Pokemon cheered for it. In subsequent encounters with its former trainer, Chimchar grows from fearing him to being determined to prove how powerful it's become. By the time it evolves into Infernape, it's completely healed of its mental scars and is one of the strongest Pokemon Ash has ever had.
With his most interesting and capable team of Pokemon, Ash's trademark creativity in battle was at an all-time high. The rapidly improving trainer was able to find inspiration from Dawn's contests when he created what later became known as his counter-shield. Allowing his Pokemon to simultaneously defend from all sorts of attacks while dealing damage, he left one trainer after another completely flummoxed. He even combined Pikachu's Volt Tackle and Iron Tail into one devastating blow.
Ash's performance in the Sinnoh League showed just how far he had come when he opted to utilize all of the Pokemon in his arsenal, rather than stick solely with the ones he caught in Sinnoh. He made quick work of his opponents en route to his rematch with Paul. The three-episode-long battle, the first time Ash ever defeated his greatest rival, was one of the most emotional victories of the entire series.
The strength of Ash's convictions was on full display during his Sinnoh journey. His victory influenced Paul to be kinder to his Pokemon, his dedication inspired Brock to chase his own dreams, and his ceaseless support of Dawn in her contests gave her the confidence to overcome adversity. Ash's infectious enthusiasm -- in addition to his newfound prowess as a trainer -- was much-needed proof that though it may not always be pretty, he was on the right path.
It seemed inevitable that Ash would finally become a League Champion after so much incredible character growth. Unfortunately, Pokemon couldn't help but shoot itself in the foot by forcing his greatest adventure to end with a Legendary thud. In the Semi-Finals, Tobias, aka Action Replay Man, used his Darkrai and Latios to steamroll the show's hero and spare the world from Ash's success for a little while longer.
