In the modern superhero era, Arrow was the show that sparked the genre into life on television.
While the Marvel Cinematic Universe and Chris Nolan's Dark Knight Trilogy were both killing it at the box office, it took the arrival of a show based on a more B-list comics character to kickstart the revolution on TV.
With two years of dark, grounded action, the show spawned an entire Arrowverse of comic book hits, and to an extent paved the way for hit Marvel series Daredevil.
Despite this start, Arrow completely fell behind the pack. Over Seasons 3 and 4 the originator of the trend went out-of-style, delivering bleak episode after bleak episode, and getting weighed down in everything from soap opera relationships to magical villains and everyone coming back from the dead.
Season 5, then, was make-or-break. Arrow had long lost critical approval, and the trust of its fans as well. Had this year not turned things around, then the show might not have seen a Season 6. While it hasn't managed to prevent a ratings slide (though Arrow has never been huge in that department anyway, they are the lowest they've ever been), it has made a great turnaround in quality.