Misty vs Ash at the Whirl Cup. It was good until Psyduck came out. To much of a plot convience that Ash forgets Psyduck's headache crap when he countered it with Bulbasaur in his rookie days. Cheapens Misty's win.
Ash vs Nando. I am so glad Nando didn't win the GF after seeing this battle. The Coordinators of Sinnoh would have been a joke.
Ash vs Wattson. Forgettable Pikachu power battle.
Dawn vs Jessie at the GF. After Zoey vs Nando's amazing five star contest battle how do you feed us this? Ursula should have been in Jessie's slot or it should have been Kenny. This one contest battle really brings down the grade of the Sinnoh GF for me. I can't believe they wasted a top 4 contest battle with Jessie's lame Team Rocket Pokemon that get blasted off every episode. Jessie should have cheated to try and gain a advantage or something. She didn't even use Yanmega for crying out loud! Worst contest battle in the series. Sucks that one of Dawn's only two victories in the GF was against such a unworthy opponent. Way to show your regional girl is actually strong writers. Okay rant over
Ash vs Erika. I listed this one because Erika is my fave Kanto Gym Leader so it sucks that her battle was so lame. Really Charmander loses because Gloom stinks? Even as a kid I thought that logic was dumb. I wanted to see Erika use her Victreebel and the battle got interrupted by Team Rocket.
Ash vs Trip when they first meet. Enough said.
There's many more but these ones come to mind first for me.
Parena or Serecham both equal Serena+Pancham!