Spongebob. Those gore scenes (the toenail) and the grossly exaggerated faces have given me nightmares. I'm never watching the show again for fear of seeing something else horrific.
Fairly OddParents had pretty bad seasonal rot, in that the people (AJ, Chester, Trixie, Tootie, Vicky, etc.) who interact and challenge Timmy are gone, replaced with cheap wish fulfillment like Sparky, Poof, Chloe.