Spoiler: It’s all of them. All of the shows.
Rugrats
This is probably one of the more well-known ~it was all a dream~ fan theories. It goes like this:
Angelica was a lonely child, neglected by her parents, and dreamed up the rest of the Rugrats to keep her company. But that's not the darkest part – the babies were all real at one point, they just tragically died. Chuckie died in a car accident, which explains his father's neurotic behaviour. Tommy was stillborn, which drove Stu to madness and is why he spends all of his time in the basement, making toys. Phil and Lil's mother had an abortion, and Angelica never knew the sex of the baby, so she invented identical twins, one male, and one female. Kimi was taken away by social services because her mother was a drug addict. The only real baby was Dil, which is why he was the only baby who didn't talk in the show.
Don't worry if you're totally horrified by this theory, though – the show's creator, Arlene Klasky, told BuzzFeed last year that it's definitely not true.
Nickelodeon
Friends
A couple of years ago, a Twitter user came up with a theory that said the entire plot of Friends was just a delusion of Phoebe's meth-addled mind. According to the theory, Phoebe is a homeless meth addict who sleeps on a bench in Central Park and watches the rest of the gang through the window of the coffee shop, imagining that she's a part of their perfect lives. Their names aren't Joey, Chandler, Ross, Monica, or Rachel at all – those are just the names Phoebe's imagined for them, and they also have completely different personalities.
The series would end with them walking past Phoebe on the street and referring to her as "the crazy lady who always stares at us". She then goes back to Central Park, where she falls asleep on a bench in front of the iconic fountain. It starts to rain, and she puts up six brightly-coloured umbrellas.
NBC
The Walking Dead
In the grand tradition of zombie apocalypse stories, this one started with its central character waking up from a coma – so, of course, there's a fan theory claiming that Rick has been in the coma this whole time, and the apocalypse is all just a figment of his imagination. It all began when people on Reddit started questioning the claim that Rick would have survived in a coma for 46 days (after the apocalypse began and before he woke up) without water or general care.
Fans have speculated for years that the series will end with Rick either waking up from his coma or having his life support turned off with Lori, Carl, and Shane crying at his bedside – but this, too, has been debunked by the show's creator. So instead let's focus on the batshit theory that Rick "died" while he was in the coma but had already been infected by the virus – it affected him differently because he was in a coma, so Rick is, in fact, a conscious Walker.
AMC
Buffy the Vampire Slayer
This one is all thanks to the season six episode "Normal Again". In it, Buffy is the victim of a demon who makes her believe she is confined to a mental institution and that all of her friends – and her status as the slayer – are imaginary. Fans have theorised that this episode alone shows Buffy in her true state, and the rest of the series is just in her head. In "reality", Buffy's mother is still alive and married to her father, Dawn doesn't exist and is actually just a figment of Buffy's imagination (which explains why she just appeared out of nowhere in season five), and her life as the slayer is simply an escapist delusion. The end of the episode sees Buffy "choose" the Sunnydale reality, therefore submitting to her delusion for good.
Joss Whedon, the show's creator, has agreed that this episode leaves the entire series up for interpretation, but personally doesn't subscribe to the theory, and thinks Buffy's life as the slayer was the "real" world.
The WB