Banjo Kazooie.
Ocarina of Time and Mario 64 are also legendary, and more influential, but BK to me represents a near perfect package.
Memorable worlds and characters, tons of variety and charm, great music, some of the best visuals on the platform, and a deeper and more refined take on the template Mario 64 pioneered.
For me it's the epitome of what made that generation of gaming so mind-blowing at the time; that leap forward into a whole new world of three dimensions that came to life like never before.