Technically speaking, there is no right or wrong way to react to the sentence “Upcoming two-part The Flash/Supergirl musical crossover episode”. However, if your reaction doesn’t involve screaming yourself hoarse with terror inside a cupboard that you’ve deliberately bricked yourself into, there’s probably something wrong with you. Musical episodes of non-musical TV shows are the devil’s anus, and the statistical likelihood is that March’s The Flash/Supergirl episodes will further reinforce this notion.

Because the world is full of terrible musical episodes, isn’t it? They’re just episodes that have inexplicably decided to jettison everything you love about the show in order to shout exposition for a needlessly lengthy amount of time. The Scrubs musical episode wasn’t very good. The Psych musical episode wasn’t very good. That 70s Show’s musical episode wasn’t very good. Oz’s musical episode wasn’t very good. Fringe’s musical episode wasn’t very good. I could go on. Grey’s Anatomy. House. 7th Heaven. Ally McBeal. How I Met Your Mother.

In fact, you can count the number of non-terrible musical TV show episodes on one hand. So that’s what I’ll do.

5. Xena: Warrior Princess, The Bitter Suite

Even when it was firing on all guns, Xena was always a hokey slab of titillation with meagre production values. On the plus side, this meant that it could pull off something as hokey, titillating and cheap-looking as a musical with aplomb. The styles meshed so successfully that the episode was nominated for two Emmys, and another musical episode was attempted two years later. That was about Xena entering a Battle of the Bands competition, and was awful, and that’s why it’s not in this list.

4. Futurama, The Devil’s Hands are Idle Playthings

Originally planned as the final-ever episode of Futurama, The Devil’s Hands Are Idle Playthings was almost unbearably poignant. Fry tries to win Leela’s affection by mastering an alien instrument, but it’s beyond him. He does a deal with the Robot Devil to help him improve, then stages an opera about his love for Leela. But it all goes wrong and, in the end, he has to salvage the day with a borderline-atonal – but breathtakingly sweet – solo. In a fair and just world, Futurama would have never returned and this would have been its final gesture to the world.

3. Comedy Bang Bang: Casey Wilson Wears a White Lace Dress and a Black Blazer

It was inevitable that fake talkshow Comedy Bang Bang would stage a musical episode, since it gleefully plundered from almost every genre of entertainment imaginable. But even when it breaks its back trying to be insincere, Casey Wilson Wears a White Lace Dress and a Black Blazer – which included songs about topical monologues and bungled interviews – cannot hide the care that went into its production. The episode’s MVP? Sir Andrew Lloyd Webber, who performs a short rap about Thailand.

2. Buffy the Vampire Slayer: Once More With Feeling

Usually held up as the greatest ever musical episode of a non-musical series, Once More With Feeling has an astonishingly high sheen to it. Not only is it rammed to the gills with musical numbers – there are 14 of them in total – but barely any of them stray from the storyline. In fact, the plot (a demon forces everyone to reveal their darkest secrets) actually helped to crack the entire series open, forcing subsequent episodes down paths it would have otherwise been unable to tread. A true accomplishment.

1. It’s Always Sunny in Philadelphia, The Nightman Cometh

But not as much of an accomplishment as The Nightman Cometh. It is not only the greatest-ever musical episode of a television show, but it’s also the greatest-ever episode of It’s Always Sunny in Philadelphia. It was so successful, and funny, and memorable, and immediately iconic, that the cast actually toured it as a stage musical in its own right. Footage of one show, at the Troubadour in LA, is available online, and sees the cast performing in front of a crowd so utterly devoted that it even manages to lose its mind at a harmonica solo. Always Sunny recently made a second musical episode, entitled The Gang Turns Black. It was great, but it wasn’t The Nightman Cometh.