Season 5 of Aqua Teen Hunger Force. Largely after the writers were given free rein contentwise, meaning we got a ton of overtly gross-out storylines with rotting corpses and severed penises, as well as Master Shake microwaving a cat.
Season 3 is the beginning of the show being less consistently good, and Season 4 is when the plots, on average, became much more visceral and dark.
Season 3 is generally hated for wasting many good plots, like the Stella vs. Chimera feud, Aisha's arranged marriage, and the possibility of seeing the girls' home worlds (we only saw Andros, Solaria and a bit of Linphea). There's also Valtor's massive Badass Decay.
Even fewer fans liked the "Vacation in Europe" side-season, which had cheap animation, had loads of They Just Didn't Care when it came to European culture and history, and barely fit in with the series' continuity.
While we're on Ninja Turtles, season 6 is generally considered the weak point of the 2003 series, due to being much Lighter and Softer than previous seasons, sending the Turtles to the future for no apparent reason, introducing a supporting cast member in the form of April and Casey's Child Prodigy great-grandson, and, oh yeah, the fact that, on the order of the toy company executives, the writers were forced to skip directly from season 4 to season 6 due to the executives feeling season 5 wouldn't sell toys as well. This was particularly painful because season 5 contained the resolution of the show's Myth Arc. And once season 5 did finally come out to much acclaim, the executives' orders seemed even more nonsensical; wouldn't characters like the Acolytes, the true forms of the Foot Mystics and Ninja Tribunal, the Tengu and his demonic minions, and the Turtles' dragon forms have made great toys?
Season 6 of South Park, largely due to the backlash against Parker and Stone retiring Kenny and their plans for Butters being the new Butt Monkey being changed by Comedy Central and fans rescuing Butters from the Scrappy heap. Needless to say, ever since that season, Parker and Stone have openly threatened to quit production of the show (to the point that Parker almost bailed entirely midway through season eight).
Fans also say that seasons 12 and 15 are suffering rot as well, the latter for deliberately derailing the characters for a melodramatic two-parter that ultimately went nowhere.
Seasons 12 onwards have gotten this from many fans to the point of where some people actually hate everything from Seasons 5 and after for changing the character's personalities, becoming more political and almost feeling like an entirely different show.
Season 17 appears to be the final nail in the coffin for many fans for amping up(for the most part) everything that people hate about the most modern seasons (Having a pop culture parodies in every episode, the worst derailed traits of characters amped up to 11, having mean spirited storylines and feeling stale). It's gotten to the point of where Trey and Matt have featured plots in episodes that could have only worked in 2008.
Code Lyoko fans consider Seasons 3 and 4 (or if not that, just Season 3) to be inferior to the first two story-wise (though far superior stylistically). While Season 2 involved and ended with an exploration into the computer's past and the progression of the major romance arcs, the next two seasons shunted that to the side in favor of episodic filler, which was more often than not dedicated to the increasingly-unfunny escapades of the comic relief characters, particularly Jim.
Total Drama's second season, Total Drama Action was received quite poorly by its viewers, half the original fan base of the original Islanddon't watch it at all. Why? Well, for starters they removed almost half of the original cast, including popular characters (at the time) Cody and Noah, completely flanderized the remaining characters, kicked off most of the fan favorites (again, at the time) such as Gwen and Bridgette early, oversaturated Owenagain, and had Chris become such a huge Jerkass that it wasn't even funny anymore. It was considered by many as the black sheep of the franchise prior to All-Stars and it seems that even the creators feel this way since almost all of the plots and characterizations (aside from Gwen and Trent's breakup which was the result of Executive Meddling, Chef's secret alliance with DJ, as well as Geoff's brief derailment into being a Chris clone) have not been referenced since that season ended.
The third season World Tour got hit by this to a lesser extent (though there are some who claim it as the show's worst) due to repeating almost all of the same problems that Action suffered from as well as creating a few new ones such as the introduction of characters like Alejandro and Blaineley and the highly controversial love triangle between Duncan, Courtney, and Gwen. The latter was allegedly done to make Fan-Preferred Couple Gwuncan canon which lasted almost all of the second half of the season, at the cost of causing what is possibly the biggest Ship-to-Ship Combat in the history of the fandom. As a bonus, it also included/started a rather pointless "Gollum Ezekiel" subplot that seemed to only serve to troll Ezekiel fans, which did not go well considering his lack of focus earlier in the series and how this season had implied to finally bring him to light.
Due to the events of the post-merge episodes (particularity the last three), All-Stars got hit by this just as bad as (if not worse than) Action making it the new black sheep of the franchise. Fan reception proved so negative that Fresh TV came forth to request fan feedback on the season.
Ben 10: Ultimate Alien, on the other hand, got the Seasonal Rot going in its second season, with tons of filler episodes, characters further acting like idiots, and a convoluted main arc plot involving the Forever Knights and an Eldritch Abomination that took a hell of a time to come out. It's commonly thought that it got better toward the finale, where Vilgax finallygot out of his decay and was portrayed as a Magnificent Bastard, but it's still controversial.
Season 4 was this when it first aired, but is now considered by many fans to be the show's best season (Mainly by people who started watching when the show returned).
Season 3 was also this when it originally aired and for a few years, but is now considered to be a part of the show's golden age by many fans.
Seasons 5, 6, and 7 of Sponge Bob Square Pants have the least fans, due to the show never the samethe same from that point onwards. note (When the show was  in 2004, due to the box-office grossthe featurefilm adaptation (which was the intended Grand Finale to the show-Season 3 ended production back in 2002, until Nickelodeon decided to revive the show) made in addition to the high ratings the show maintained, creator Stephen Hillenburg notably changed his position in the show's production and co creator Derek Drymon left.) After Season 4, the show became much more grotesque compared to the earlier seasons-Season 5 and 6 have Vulgar Humor, massive flanderization of various cast members (with some being flanderized into ditzes and designated heroes), occasional dark humor, creepyNausea Fuel, and overall less charm; while Season 7 was Season 6 taken Up to Eleven and on top of that was also Darker and Edgier compared to past seasons. Season 7 also featured OneCoarseMeal, which is considered one of the most hated episodes in the series.
Season 4 was also this when it began airing, but is now considered by many to be the golden age.
The Simpsons: Fans of the show have wildly varying opinions on which seasons were good and which ones sucked, but these are considered the worst:
You won't find too many supporters of Season 11.note (For reference, this is the season that killed off Maude Flanders, had Barney give up drinking, made Apu and Manjula the parents of octuplets, and presented whimsically self-referential episodes like "Saddlesore Galactica", "Missionary: Impossible", and "Behind the Laughter".) It was the least grounded and realistic of all Simpsons seasons with more out-there plots and twist endings. Seasons 10 and 12 also suffered from this, but to a lesser extent than season 11. Seasons 13, when Al Jean returned as a showrunner, was a partial return to the days of the less wacky Simpsons. However, these later seasons are often criticised for being when The Simpsons became a Franchise Zombie.
Season 9 gets this too, due to "The Principal and the Pauper" (the infamous episode where Principal Skinner is revealed to be a street punk named Armin Tamazarian who took on the identity of a man named Seymour Skinner so he wouldn't have to tell the man's mother that her son was M.I.A), one of the most hated episodes in the entire series (even the show's creators hated it). Add to that the parade of celebrity cameos and humor getting cruder and more sadistic to compete with South Park, as well as the induction of Mike Scully as showrunner. The season still features a lot of classic episodes, especially the holdovers from the Season 7-8 run, but the ratio of bad to good spikes dramatically and never really gets back in balance. When fan reviewer Mike Amato covered Season 9, it was the first one in which he was able to name five episodes he hated.
Futurama is considered by many to be going through this ever since it made the Channel Hop to Comedy Central. This is largely because of all the Anvilicious episodes that basically turned the show into a cleaner South Park, lampooning current issues with ham-fisted attempts to show the writer's side of the argument (the episode "Proposition Infinity," an obvious Expy for gay rights, comes to mind).
More criticisms include the heavy Flanderization of several characters and a slight increase in Nausea Fuel that still existed but wasn't anywhere near as noticeable in the Fox episodes.
Futurama also has an in-universe example in the form of Everybody LovesHypnotoad, which has been going downhill since its third season. note This is according to someone who's immune, or at least highly resistant to, the Hypnotoad's brainwashing thanks to being his own grandfather
Teen Titans fans generally consider season 3 to be this, since they changed the Big Bad from the awesomely creepy Slade, who was Robin's archrival but still had a personal beef with the rest of the Titans, to Brother Blood, who started off perfectly menacing but spiraled into Villain Decay quickly and had limited interaction with any of the Titans besides Cyborg, and having a weak story that only got two episodes and a two-part finale worth of exposure when other arcs usually had one or two more.
And, to a lesser extent, Season 5, probably due to it coming directly after the extremely well-received Season 4. It's still generally accepted, though, mainly due to its awesome Grand Finale (the two-part final battle, not the controversialMind Screw of an actual last episode.)
To some, season two of X-Men: Evolution is this. Over focus on romantic side plots, less focus on their battles and less action. Thought the ending managed to fix that by revealing The Masquerade, leading to it Growing the Beard in season 3.
During the third and fourth seasons of KaBlam!, the production company for Sniz & Fondue went bankrupt and more one-shot shorts were produced, with mixed to negative receptions among the fandom, though the remaining regular shorts were claimed to be even better that season.
Seasons 4 and 5 used to be this while they were still running. Now they are considered to be a part of the show's "golden age" by many fans.
Phineas and Ferb is still popular, though there are many people who think that the show's quality has been going downhill during season 3. More accurately, the episodes from the middle of the third season just haven't been as good as episodes from the second season and the first half of the third. That said, the show is still quite good - it just seems to have peaked in quality sometime around the Big Damn Movie, Phineas and Ferb The Movie: Across the 2nd Dimension.
Some people actually cite the downfall of the show as far back as Season 2, due to more emphasis on formula and redundant jokes as opposed to the show's first season which was a Sliceof Life show mixed with comedy. In other words, it was Family Guy meets Hey Arnold! in it's first season. Ironic that Dan Povenmire and Jeff Swampy Marsh pretty much wrote every episode of Season 1 and wrote less for the show after that.
A few fans felt the show got stale around the time Season 3 came around due to more emphasis on the formula and the writers had stopped developing the characters. Coincidentally around this time, many of the show's original writers left to work on Regular Show and Adventure Time.
Among Scooby-Doo fans, general consensus is that two series fit this. Firstly, The Richie Rich/Scooby-Doo Hour, which made the bold move of completely changing the show's premise and in doing so fell flat on its face. Fred, Daphne, and Velma were inexplicably gone, the series was switched to a Three Shorts format, and the mystery-solving plots and Scooby-Doo Hoax were jettisoned in favor of slapstick chase scenes featuring Scooby, Shaggy, and Scrappy running away from real monsters for five straight minutes (the real monsters, incidentally, don't seem to have been part of the problem, as laterinstallments featuring real ghouls were received much better). Second, Shaggy & Scooby-Doo Get A Clue!, which also tossed out the mystery premise and Fred, Daphne, and Velma, but not only that, got rid of the distinctive Hanna-Barbera art-style and horror elements entirely, in favor of, of all things, a spy thriller. The Scooby-Doo and Scrappy-Doo Show sometimes gets lumped in there too for introducing... well, guess.
While The Avengers: Earth's Mightiest Heroes has several great episodes in its second season, it also suffers from having more filler than the first, especially after Jeph Loeb and Man of Action Studios came on as executive producer and creative consultants, respectively. They created so much filler, that subplots the original writers set up earlier went unresolved and/or unexplored by the time the show ended. Their run also saw most of the Avengers get pushed Out of Focus, and Out of Character Moments become more frequent. Plus, the animation in this season sometimes seems cheaper, and the awesome theme song permanently got ditched in favor of recaps of old episodes and a promo for the Avengersmovie.
Many King of the Hill fans consider season 9 the show's nadir, due in large part to the massive retcon of Peggy in the premiere "A Rover Runs Through It" and Lucky becoming a regular cast member ("Care-Takin' Care of Business"). These along with ongoing Flanderization (especially of Luanne) and a lack of well-regarded episodes make it a season few fans stick up for.
While hardly bad, season 3 is often considered the weak point of Metalocalypse, due to a rather severe Myth Arc stall and the series not being allowed to use its traditional Gorn due to the network fearing it would be perceived as too similar to Superjail. Season 4 was much more in the spirit of the first two seasons. The episodes were also twice as long (half-hour instead of quarter-hour), which while a good idea in theory, resulted in some serious pacing issues.
Fans of Dan Vs. consider season 2 to be this, due to too much focus on Elise, a lack of Black Comedy, and making Dan into a Failure Hero (when one of the best parts of the show was watching him succeed). Season 3 is considered an improvement, but not by much, which is why the show didn't get renewed after that.
Schoolhouse Rock: Earth marked the first time in seven years that the crew released new songs. Unfortunately, almost none of them, if any, hold up against the classic songs. When counting only the seasons that aired on ABC, Scooter Computer and Mr. Chips would get deemed the weakest. It didn't even get a home video release until Disney started releasing Schoolhouse Rock songs on DVD.
Robot Chicken has two fan-noted cases of this: Season 2, which is considered weaker than the sublime first season (though fan opinion of it has gotten better over time) and the infamous Season 5, which saw the show's usually witty humor go completely out the window in favor of over-the-top violence, crude Toilet Humor, and too many Dude, Not Funny! moments to count. Apparently either the writers were listening or replaced, because Season 6 took a sharp turn in the other direction, much to the delight of the fans.
There are a lot of fans of Thomas the Tank Engine that have diverse opinions on when the series went downhill:
Some say it was when the show became less true to the Rev W. Audry's original stories. Awdry himself disowned the show following Season Three, when writers took more liberties with his stories and also started created their own, which sometimes disobeyed railway code and had inaccuracies to the story mythos. After Season Four, adaptions of the original novels ended altogether.
A lot of fans were also disappointed when Brit Allcroft stopped producing the show and HiT Entertainment took its place and when the latter converted the show's format to CGI instead of using models.
A lot of fans pinpoint Sharon Miller's run as head writer (Seasons 9-16) as the low point of the show, due to stories focused aimed towards much younger audiences and the most evident amount of railway inaccuracies. Season 17, where the series was taken over by Andrew Brenner and reverted to more mature storylines, has gotten a warmer reception thus far.
The second season of Superjail is usually vocally met with scorn and invocations of this, if not just disappointment. Criticisms ranged from the animation being "too fluid", the characters suddenly having backstories revealed, some changes with the characters' personalities, and the story formulas changing. Most notably, the episodes no longer all had wild bloodbath sequences, and some had a pun or a spoof Aesop tacked on to the ending.
While Season 3 is looked at a little more fondly due to the crew attempting to merge the styles of both previous seasons, the change in animation studios (necessitated by Augenblick bowing out to work on Ugly Americans) and the Warden becoming too childish are still subjects of criticism.
While the Superfriends series was probably never "good", the nearly-universal consensus is that it gradually got better as it progressed, with the first season being particularly dire. (How dire? The introduction of the Wonder Twins and Gleek was a step up.)
Gargoyles fell into this after Greg Weisman left at the start of Season 3. Xanatos and Fox became full good guys and every villain was a Card-Carrying Villain with little to no depth.
Season 3 of My Little Pony: Friendship is Magic was generally not as well-received as the prior two. Since it was only half the length of an average season (likely to hit the 65-episode syndication minimum), its problem episodes - which, naturally, vary depending on who you ask - tend to stick out more. Almost a full third of the episodes therein have been a point of contention for at least some fans. (Especially Spike at Your Service, which had to be drastically rewritten, resulting in the final product being lazy and rushed.)
This was the first season without Lauren Faust's involvement. She remained significantly involved with Season 2.
Season 3 has also been criticized for pacing problems, largely due to trying to pack in alot more ambitious and eventful plots has frequently led to criticisms regarding time and padding issues, with even some of the better regarded later episodes criticized for story pacing and not getting the correct development (eg. "Keep Calm and Flutter On's" noticeably rushed resolution for Discord's Heel-Face Turn, and the season 3 finale, "Magical Mystery Cure", for the same thing, except with Alicorn Twilight.).
Batman The Brave And The Bold ended with its third season due to the show's creators wanting to avoid decay. That said, the third season is still considered to have taints of this trope, due to contention about the quality of the plots and Batman occasionally veering from The Ace to Canon Sue.
While not as much as the uncancelled SpongeBob episodes, when people say Rugrats went through this, it leads to a Broken Base. Most people say it's either after they added Dil or Kimi, some people say after it got Un-Cancelled, due to more emphasis on Toilet Humor and not as much Getting Crap Past the Radar and due to Arlene Klasky not letting Paul Germain (the showrunner of the first 3 seasons) know that the show was coming back and the show lacking his influence.
While not talked about as much as Rugrats, most fans of All Grown Up! say that it declined after Season 2 due to more drama and less comedy, Flanderization, an Art Shift, and more unrealistic plots.
American Dad!, while not as much as Family Guy has been said to have a small/minor decline in Season 7 for less comedy and more drama as well as some character's traits becoming more exaggerated (especially Roger). The latest season has provoked many mixed reactions.
It's also disliked for having boring plots with glacier-slow pacing and having most of the actual comedy reduced to over-the-top gore and bizarre big lipped alligator moments.
Some fans say that the rot began in Season 5 and see Season 7 as a worse version of the aforementioned season for the reasons already mentioned.
The Dreamstone's first season is often better regarded due to its darker plotlines and character designs which are more loyal to Mike Jupp's original concept (along with suffering less from Executive Meddling forcing more side characters). The later seasons however do get credit for aspects such as more fluid animation and remedying some of the earlier episodes' flaws (eg. giving the heroes more focus and downplaying their Unintentionally Unsympathetic facets against the Urpneys).