[chevluh]

I'd say GTA 3.

Even though its core mechanic was pioneered before by the previous GTAs, or partially by other, older stuff (Mike Singleton's stuff or Hunter on amiga come to mind, you could tell they were trying to do something big, though you couldn't quite tell what because of the technical limitations), GTA 3 was the one that made it appealing to a broad audience, the one that made it click together.