If you want to get really specific, Halo 5 was the Halo Killer. 343 and Microsoft ruined Halo, no other game did it. People wanted Halo to be good so badly. They had 3 tries, and their only successful release was a RE release of Bungie games with the MCC lol.
Halo 4 wasn’t awful but left a sour taste with fans. But there was goodwill left, and we were waiting Halo 5 to be that redemptive title. That never happened.
If you want to assign a specific external game as the Halo Killer, then it is Destiny.
Even with all of its server issues, it is the only successful MMO FPS and does PvE better than every game I’ve played not named Final Fantasy XIV.
Further, it has the best gunplay outside of Call of Duty, is the 2nd most popular FPS on the market right now, fills the scifi FPS niche that Halo dominated, and is made by the same devs that made Halo.
While there’s no correct answer here, I think the most widely beloved OSTs of all time probably belong to the Final Fantasy, Donkey Kong, Legend of Zelda, and Super Mario IPs.