Honestly, the Unova league was far more disappointing. The Kanto League was understandably underwhelming and less robust when it comes to battles, but it really did fit the narrative quite well as it was served as a means to show Ash that he still had a ways to go and his reason for his failure was entirely his fault. He was complacent, cocky, and was inept in truly bonding and understanding his pokemon (Charizard), so while the odds were against him, he didn't deserve to win and his loss at the league contributed towards his own development as a character and a trainer.
The Unova League had better battles, but it was poorly paced. Much of the tension was lost from the get-go due to a very underwhelming battle with his main rival with what ended up being a sluggish 1v1. To make matters worse, the writers practically shattered any sorts of hope, faith, and attachment to Ash by having him lose to one of the most idiotic trainers in the anime which is a kick in the balls to whoever became emotionally attached to Ash's journey for all of these years. I definitely can forgive the writers for Ash losing 4 years because of his incredible improvement as a trainer where it really felt he getting closer towards reaching his goal. However, I cannot forgive this where the writers shat on my commitment towards Ash by having him lose to Cameron and ultimately cause me to lose faith in the series at the time.
Unova League was much worse. No contest. It didn't have any redeeming qualities other than Ash vs. Stephan.