The Flash - which has since garnered an incredibly large fanbase and is, without question, one of the most popular TV shows the genre has ever produced.
Arrow achieved a great deal of success in its early days, bringing in impressive viewing figures for its network when its flagship show Stargirl.
But what makes The Flash the enduring Arrowverse juggernaut that it is? The truth is that there are many answers to that question but one of the primary reasons for its popularity is down to the fact that, in many ways, The Flash feels like the definitive comic book TV show, personifying the adventures the titular hero would have on the page in live-action. On the surface it matches the tone of the comics, adopting a more light-hearted approach than its predecessor, but underneath all of that, it also stays true to the source material, adapting some of Barry Allen's most famous storylines over the course of its run and utilizing its own powerful storytelling methods to highlight why Barry himself such a rich character.
Arrow's constant deviating from the comic books was one of the many reasons for its sharp decline halfway through its run - with controversial decisions like killing both Black Canary and Amanda Waller alienating comic book fans - but The Flash regularly ensures that it pays tribute to the character's history as the writers stick as close to it as possible. Granted, there are some deviations but not many which risk offending its core audience - and that results in a much more enjoyable property for fans of the source material. While some of the other shows attempt to do the same, the light-hearted tone, familial storylines, and fun, gimmicky villains on The Flash make the show more accessible to people of all ages. And this is not something that every modern-day comic book show - or even every Arrowverse show - can claim to be.
Arrow reminded TV viewers that the small screen was a great place for superheroes but The Flash took that lesson and ran with it, giving audiences one of the most enjoyable superhero shows of all time and the Arrowverse a show so successful that nothing that came before or after it can match its levels of popularity. And when you factor in that the Scarlet Speedster is probably the most well-known superhero the Arrowverse has adapted in a leading role - before Superman & Lois, that is - and the fact that The Flash is arguably the quintessential comic book show, it's easy to see why that's the case.