The Simpsons: Principal and the Pauper - There's plenty of other bad Simpsons episodes I've seen over the course of this show's run, but I consider Principal and the Pauper one of the first signs that something was going terribly wrong with the show. It's a plot that ultimately ends up wasting itself in it's own 22 minutes by having Skinner keep his fake name as a way to play on status quo.
Danny Phantom: (tie) Splitting Images/Reality Trip - Actually, there's at least two episodes in Season 3 that are worse off--waaaaaaaaaaaay worst off then these two, but for spoilers sake, I won't say it, so for the time, "SI" which is just so blatantly cynical (even for me) that poor Danny keeps getting chewed out while the main "foe" didn't even get his own comeuppance.
As for "Reality Trip", well, the episode is decent enough for your average action summer flick feel, but it got completely ruined with any lack of Valerie (which considering what the plot was about seems almost illogical NOT to have her) and the deus ex machina/reset option that peeved me off SO greatly. At least the one in "TUE" is justified with the Jazz revealing angle (for me anyways), but this? I expected better from DP.
Digimon Zero Two: The ending (forgot the title name) - This ending just KILLED the second season of Digimon for me, not that Season Two was good to begin with; before, it was a contrived mess of random plots that seem to hang together with a very loose piece of string, but the ending and it's future of the Digidestined gave off a serious "What th--" mode with roles that doesn't even fit most of the characters and came ridiculously out of the blue. It just placed the final nail in the coffin.
Xiaolin Showdown: The Demon Seed - Their enemy is a talking plant...with a french accent. My head exploded.
Avatar: Crossroads of Destiny - Unless the creators have a GOOD reason why Zuko ended up the way he did by Season Three, I am not pleased with the decision he made--not happy.
Fosters: Bloo's Brothers - It's bad enough I have to hear this jerk shove it in people's face that he's going to the Ice Charades over and over when they can't go because they don't have tickets, but even more so when he complains by the end by the time Mac chose the real Bloo out of the fakes. It's any wonder why I dislike him SO immensly.