Okay, I've posted here before, but here's my definite opinion.
SpongeBob was almost consistently good (there was always the occasional I'm With Stupid) up until the movie. Seasons 4 and 5 have some good episodes. Some of the best ones (like Krusty Towers or Friend or Foe) can be compared to the first three seasons. But the worst episodes (like Waiting and Good Neighbors) can be considered some of the worst episodes of the show. These seasons can be good, but also have some bad ones. Seasons 6 and 7 are infamous for having a lot of hated episodes (House Fancy, The Splinter, A Pal For Gary, One Coarse Meal, and Stuck In The Wringer are some of the most reviled episodes of the show just to name a few), with almost no redeeming episodes. If the show wasn't disgusting, it was unwatchable. If it wasn't unwatchable, it was way too mean spirited. If it wasn't mean spirited, there's probably some other reason it was awful. And if it wasn't awful, there was just nothing. Episodes would just have absolutely no substance. There was a very big lack of good episodes. Seasons 8 and 9 (especially the latter after the sequel movie) are definite improvements, but not perfect (Little Yellow Book). Everything after that is just as hit or miss, but there, the best episodes are a lot better (like Mimic Madness or Bulletin Board). I don't really know what the show is like now, but there might still be a light at the end of the tunnel. I doubt it would be like the early days, but anything's better than seasons 6 and 7.
With The Fairly Oddparents, it was good until Season 6. The worst episode up until them was It's A Wishful Life. It was still watchable by and after season 6, but the show started introducing new characters seemingly on a seasonally basis. Poof wasn't that bad though. But nothing after season 8 is bearable. The last two seasons of the show are even worse than SpongeBob seasons 6 and 7, because at least there, there was the occasional watchable episode. Not good, but watchable. That's not the case here. Sparky makes season 9 unbelievably obnoxious, and it's the same with Chloe and season 10. It's not funny or entertaining or tolerable anymore, just depressing remembering what it once was. By the last half of season 10, they start using Flash animation, so now, instead, it looks like Total Drama or the show about a… beast that Butch Hartman also made that I refuse to mention. I guess by this point, Butch Hartman just lost his charm in his writing, since it used to be good, and then after this show concluded he went to work on the show I still won't name. Speaking of concluding, the ending to the show is terrible and not at all satisfying. When I was watching it after it first aired, I wondered if they just had more episodes to release. That's how bad of a conclusion it is. The only good thing about it is that they euthanized the show when it was clearly suffering and needed it.
In conclusion, I think SpongeBob is better. SpongeBob's first three seasons are some of the best Nick has to offer, and even after that, there's still a good amount of good episodes and ones that capture the charm of those first three seasons. There may have been potholes (more like pits to hell in some cases) along the post movie road, but ultimately, the show still had a respectable run, and who knows? Maybe the show still has some of those episodes that capture its early glory. Meanwhile, with the Fairly Oddparents, it was up there with SpongeBob, Rugrats, Rocko's Modern Life, Hey Arnold, and more as one of Nick's best shows until season 6, and still managed to stay watchable until the second Sparky is introduced. But both shows are there to show how good simple concepts (like working at a fast food restaurant or being able to have almost any wish you want) can be made creative in the right hands, and with the right writing.
