Here's just a few in no particular order:
Ash vs Blaine (Initial Match)- Yeah, yeah. "Pikachu, the horn!" But there's more. Ninetales takes down Squirtle with FIRE SPIN? Seriously? Oh, and Magmar's "immunity" to Pikachu's Electric-Type attacks. I never bought that for a second. Overall, a real let down of a match. And it's so ironic, because Ash's rematch with Blaine (Charizard vs Magmar) was easily one of Kanto's most epic and memorable duels.
Ash vs Claire- Not so much the battle itself, which was fine. I was just always annoyed that Ash had to resort to using Charizard and Snorlax for this one. Brock even mentioned that Ash didn't have faith in his Johto party to take care of things. The writers OPENLY acknowledged how much Ash's Johto team sucked on its initial journey yet did little to change it. Sigh.
Ash vs Wattson- Hax. Hax. Hax. Totally unnecessary. Nothing else needs to be said, other than it gets my vote for the absolute WORST Gym battle in the anime. Period.
Ash vs Brandon (Charizard vs Dusclops)- Again, been mentioned to death, but Ash must have suffered a recent blow to the head. No Fire-Type moves AND using Seismic Toss? Seriously? Totally forced loss to stack the odds against Ash and make Bulbasaur and Squirtle the stars.
Ash vs Candice (Gliscor vs Snover)- Pathetic. Why was Gliscor even used here? After its epic victory over Byron's Bastiodon, I was convinced it had overcome its status as Ash's "baby." I was wrong.
Ash vs Tobias (Torkoal vs Darkrai)- Torkoal's power is unbelievably bipolar. Manages to hold its own against Brandon's Registeel, and even bring that beast to its knees! Yet it can't dodge ONE Dark Pulse which it faints to. And it's a shame too, Torkoal was easily Ash's best bet for a Fire-Type against Tobias. Quilava had only recently increased in power, Infernape was probably just wiped out after facing Paul, and Charizard couldn't have made it in time. Completely forced loss.
Iris' Dragonite vs Georgia's Beartic and Dawn's Mamoswine- It must be nice having a Dragonite with an immunity to Ice-Type attacks.