I assume it must have been Pokemon Gold. I only ever beat the E4 though, and never progressed much through Kanto. But that's considered postgame anyway.
I was pretty bad at actually completing games when I was a kid. I'm pretty certain I beat Super Mario 64 as well, but not with 120 stars. I came back years later to do that.
Ah, yes, the Negotiator:
General Kenobi
<sneaky beeping>