My all time favorite Ash league battle is Charizard vs. Harrison's Blaziken during the Silver Conference. I remember that battle being one of the most exciting things I've seen in Pokemon as a kid and I still think it's a great battle. I also liked Ash's battle against Gary in the same league.
The worst ones for me are a tie between his Kanto battle against Ritchie and his freaking battle against Cameron in the Unova League. Now Kanto I'll let slide a little bit since that was the first time the writers wrote for a league, but the way Ash lost was just ridiculous. Team Rocket stealing his pokemon and almost making him late, plus the referee saying that a sleeping pokemon in unable to battle? I'm glad things have improved since then..
And Cameron's battle, I don't even need to get into that. A dumb kid with a powerful Lucario managed to beat Ash, and eh didn't even have a full team. That's like a big slap in the face after watching Ash in the Sinnoh League fighting against a Darkrai and Latios. He may have gotten beaten quickly that time too but at least he made it to the final 4 and Sceptile and Pikachu did put up good fights.