I think I can make a case for any year since
2015.
2015: Bloodborne, Helldivers, Tearaway Unfolded, Uncharted: The Nathan Drake Collection, Until Dawn
2016: Alienation, Gravity Rush Remastered, Ratchet & Clank, The Last Guardian, Uncharted 4: A Thief's End
2017: Gran Turismo Sport, Gravity Rush 2, Horizon Zero Dawn, Matterfall, Nier Automata, Nioh, Persona 5, Uncharted: The Lost Legacy
2018: Astro Bot: Rescue Mission, Detroit Become Human, God of War, Marvel's Spider-Man, Shadow of the Colossus Remake, Tetris Effect
2019: Blood & Truth, Concrete Genie, Days Gone, Death Stranding
2020: Astro's Playroom, Demon's Souls Remake, Dreams, Final Fantasy VII Remake, Ghost of Tsushima, Marvel's Spider-Man: Miles Morales, Nioh 2, Persona 5 Royal, Sackboy: A Big Adventure, The Last of Us Part II
Actually, pure numbers alone, 2020 is one hell of a PS4 sendoff and intro to the PS5.
For me, Bloodborne by itself was a game I revisited a dozen times and still will revisit, so that elevates the year by itself. Plus, Until Dawn ended up being a really fun party game in its own right. 2017 was quite strong with a great new IP and a full-length Uncharted game a mere year after 4. 8, though.... between a marvel of a VR platformer and one of the best games I've played in GoW, that's up there.
2020, though.... it's hard to deny how strong it was. A JRPG remake, a bunch of games made by regular people, two platformers, an open-world samurai game, a superhero adventure, and one of the most ambitious narratives I've seen put into a game. Despite how highly I think of Bloodborne and God of War, I also think very highly of The Last of Us Part II in all departments (gameplay, sound, plot) and thought about the narrative for a long time after finishing.
So 2020 gets my vote, but I can't begrudge someone for picking something else, whether it was the introduction of VR in 2016 or the year of the underrated and awesome Days Gone.