It was actually probably more like the 30th cooperative two-player game (think Joust and Mario Bros.), but it was the very first side-view beat 'em up of any kind, as opposed to top-down brawlers like Gauntlet, top-down shooters like Ikari Warriors, or side-view shooters like Contra.
Double Dragon became the template for pretty much every brawler that followed, not only in terms of the viewing angle but also the gameplay and large sprites that allowed characters and enemies to have lots of personality.
Double Dragon begat Final Fight, TMNT, The Uncanny X-Men, The Simpsons, etc., but also Altered Beast, Bad Dudes, Golden Axe…
Highly influential game that was really fun to play.