Perfect Dark. If you'd ask me to narrow it down to one game all-time, not just '00, it would still be PD. A gem, and I am all warm and fuzzy seeing it represented a few times in this thread.
But damn a lot of good stuff released that year. Skies of Arcadia, Shenmue (in the US), FF9, Grandia 2. Even WWF No Mercy. A great time to be a gamer.