To answer seriously... Probably orange islands. If you think about it, its the only listed time where ash was not a complete moron releasing useful pokemon. Its also the only tournament he won. He had an awesome lapras the whole season and caught a snorlax.
Although his Orange Island run was impressive, I felt like it was pretty much filler, and doesn't really count. Although to be fair Ash's strongest lineup was mostly there.
I was extremely disappointed in Gen 5 though. Ash lost to some punk he just met with only five pokemon, and the rival battle was a 1v1. The entire league felt rushed and rather boring. Gen 4 league in contrast was impressive, Ash brought back many of his old favorites and actually stood a chance of winning if Tobias didn't show up for the sole purpose of defeating Ash. (I was also disappointed that he wasn't seen in Gen-5)
I don't necessarily care if Ash wins the league or not, but for Ash to do WORSE than before kind of annoys me. I hope Gen 6 he makes it to Top 4 again, or even better yet the finals. I also feel as though the series shouldn't end with Ash winning the league, but rather, winning the league is merely the first step in becoming a Pokemon Master.
Perhaps afterwords he'll go smashing in some Elite 4/Champions or even fighting some of those past tournament winners. (Like, a Pokemon League for Pokemon League Winners)
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