Whichever game you or anyone thinks is the most important video game for PlayStation.
Nintendo and Xbox are simple to pick but as seen already lots of people have lots of different choices for PlayStations.
It's weird having "the most important" of a console brand since sales of a game could be a reason or just influence overall with the brand itself
The original PlayStation got the adult attention that Mega Drive had before. It was a combination of things. Can't really pick one since influence and sales were very close to lots of games. People say about FF VII, but the whole RPG scene was the differential of the PSX - and a lot of people didn't finish game, so... Gran Turismo was the most sold of the console, but chances are that Ridge Racer is the most loved
PS2 got the same feel, but
maybe GTA 3 could be the most of all even thou San Andreas was the most sold. I'm not really sure about that because I don't think GTA 3 would have the same impact to the industry if was a console exclusive. I really believe that modding was the real point of the franchise and this gave a domino effect, with more people talking about it, and more people getting a PS2 would have the mindset that "it needed this game, and this game, and also this game", leading to San Andreas, but didn't lead to that much people really caring about the game per se
PS3 whoever I can say Last of Us was the pinnacle of the whole generation, but
I really think that was Uncharted 2 since is a PlayStation studio from the root, the real predecessor of LoU and basically every future PS Studio [over the shoulder] game from now on, and a great game first and foremost - the first Uncharted is basically a pretty looking PS2 game. Let's face it. It's rough.
PS4... we got lots of 9/10 games. Games run better and looked better than ever, but a whole bunch of PS studio games were different versions of Last of Us. We didn't have a true landmark game of the whole generation - not just PlayStation. PS5 it's going for four years and it's even worse since all games are just PS4 2.0 games