Basically we're here to discuss some of Ash's worst losses and battles. This includes movies, tournaments, gym battles, etc. From the moment he got Pikachu to after the Unova League and everything in between.
Heck to be honest, it can include battles that he has won! Some of those wins were simply plot driven and nothing more so undeserved victories can count as his worst battles as well.
What battles make you cringe when you go back to watch them or jump into your mind first when you think about Ash losing?
For me one of the worst was Charizard losing to Brandon's Dusclops. At that point in the series, Ash should have known better than to use a Fighting type move of a Ghost type because he had battled Dusclops before against that coordinator Phantom.
Losing in the Indigo League due to a disobedient Charizard was both a fail AND embarrassment.