Gaming's Star Wars, huh? That's a tricky one. Looking over the forum, there are plenty of nominations, so to be clear, let's go over the criteria that Star Wars itself sets.
1. The most obvious, it must be very well known and talked about. Whether everyone likes it or not is both debatable and not mandatory.
2. It must have some form of cultural significance to us gamers today. That means some time must have passed after its release before it can be considered our "Star Wars." Otherwise, it may as well be just a fad.
3. While it can be selected parts of a series, it cannot be a whole series. Why? It's statistically impossible. Episodes 4-6 of Star Wars are unarguably the most thought of when it comes to the Star Wars name, but at the same time, no one is actually cheering in load voices for its spin-offs, such as the prevalent Clone Wars series, or the Force Unleashed games. Just as equally true, it doesn't have to be one game specifically, since the Star Wars everyone holds dear is actually comprised of three movies.
4. It doesn't have to be the first ever great video game. Believe it or not, there were plenty of great movies before the advent of Star Wars; they just aren't as widely acknowledged.
5. It cannot be Star Wars itself. There may have been several great Star Wars games, but I believe that there are stronger candidates out there. Besides, going with Star Wars itself is essentially a cop-out in this case.
6. It can be any game, regardless of platform. It doesn't matter if it's console or PC. Some games appear on both, and besides, gaming is gaming.
7. It doesn't have to be as widespread as Star Wars. The series has been out for at least 40 years and has longer since made it to every form of media known to man. It has had a head start, so it's unreasonable to expect any game to reach the same level of cultural relevance. This is especially true, as video games as a whole have only been around for about 30 years.
8. And lastly, there is the whole "gaming as an art" argument. Truthfully, I'm on the fence about including this as a mandatory part of the criteria, simply because of the argument of what can or cannot be considered art. So if it can be considered art, that's great, but I don't think it has to necessarily fit the bill here.
And that's the criteria I believe we need. Feel free to amend as you see necessary.
Now, back on topic, I personally think Final Fantasy VII fits best. The answer as to why should be very clear.