1. Christopher (The Sopranos)
Oh man, you know what's a good idea? Letting one of the cast members start writing episodes! I mean, they're good at acting - why wouldn't they also be good at writing television?
Anyways, Christopher was written by Michael "Christaphuhhhh" Imperioli, and good god is it bad. To be fair, Imperioli's previous writing credits (season 2's From Where To Eternity and season 3's The Telltale Moozadell) are pretty solid episodes, but that's no excuse for Christopher, aka "The One Where Everyone Is Suddenly Weirdly Into Christopher Columbus???" While there are some solid storylines happening on the sidelines (like Bobby's wife's death), most of the meat of the story is around everyone suddenly being like "I FALL INTO THIS GROUP OF PEOPLE AND HERE'S WHAT WE REPRESENT." Also, this is one of the rare episodes where Silvio has a relatively major part to play, and while Silvio can be fun, the dude really isn't an actor and isn't meant to be carrying storylines. He's more a "Bruce Springsteen band member wearing a wig who won't leave the set for some reason."
Maybe the lesson is - don't let the one guy who's character is specifically made to be a bad writer on the show into an actual writer.
2. Co-Pilot (The Shield)
Each successive season of The Shield does an amazing job at ratcheting up the tension and stakes from the previous one at a breakneck pace, but season 2 is where it really got started with the introduction of Armadillo Quintero, a violent gang leader who push the Strike Team to some pretty wild extremes that culminate in arranging for his murder while in jail. And right after that happens, when you just HAVE to know what happens next...we get Co-Pilot.
Co-Pilot is a weird, ill-fitting prequel for the entire show - explaining how Vic 'n the Strike Team got put together and how everyone found themselves in the Barn when the series began. The insane thing is that apparently Vic and Shane were squeaky clean detectives SIX MONTHS before the start of the show, but after finding out being a dirty cop paid well, they decide to become the cop-murdering Strike Team we found in the pilot.
Except this doesn't jive AT ALL with what the show set up - Vic and the Strike Team were legendary for their reputation and corruption. It was implied they had sorta always been this way, and that they had been operating for years and years. But that implied history was erased and the timeline made considerably weirder with Co-Pilot, a pointless, bad, and - worst of all - unnecessary retcon the series never needed.
3. Here Comes the Son (Gilmore Girls)
In case you weren't aware, successful shows will often have an episode referred to as a "backdoor pilot" - that is, within the context of the main series, they'll do a weird one-off episode that follows a character to a new location or situation, separate from the rest of the storyline. Basically, they're trying to do a pilot for an entirely new show but testing it with the audience for an already-popular one.
These episodes are almost universally terrible, because they're so weird and unrelated to the show you ACTUALLY want to be watching. One of the worst ones of these is Gilmore Girls' Here Comes The Son. The bad boy love interest of Rory Gilmore, Jess, travels to California to live with his biological father - and hijinks ensue! Sorta. Actually, not much happens, but what does happen is pretty boring and unappealing. Meanwhile, there's a decent storyline going on in Star's Hollow, but it's impossible to enjoy it when the show keeps moving back to Jess being mopey in some beach town.
4. Unbowed, Unbent, Unbroken (Game of Thrones)
The episode was relatively infamous almost immediately due to its ending, which saw Ramsay Snow become another in a long line of people to make Sansa Stark's life absolutely miserable. Yep, this is the one where Ramsay raped Sansa - criticized for the implication that she was being raped to motivate Theon's storyline. As it turned out, that really wasn't the case - but it WAS a pretty lazy final moment for the episode, since it tells us nothing new about Ramsay (he's mean and rape-y? HE WAS CUTTING OFF DICKS AND HUNTING WOMEN LIKE DOGS NOT LONG AGO, WE KNOW HE'S BAD NEWS), nor Sansa (her life is shitty and everyone treats her bad? Surprising!).
But that isn't the real mark against this episode (although it IS extremely lazy and not very interesting). The real black eye of this episode is the Dorne storyline.
The Dorne storyline of season 5 was pretty darn half-assed up to this point - we'd gotten very little development for Dorne as a separate kingdom of Westeros, and even less development of the new Dornish characters (primarily the Sand Snakes and Doran Martell). So when the story's climax occurs as the most lazily-choreographed, half-assed fight in the series so far, it really felt like this was all a meaningless detour for the story, with great characters like Jaime and Bronn stranded helplessly.
Maybe they shouldn't have gotten their choreography from a Simpsons Treehouse of Horror episode: