Oh my f---in' god! Those who didn't list Jet Force Gemini, DROP KICK!
It is easily the game that Ratchet, Jak, and all these other adventure games today builds on! It is one of the most amazingly detailed adventure games ever! When you get your robo friend, he has backlights that glow (like car lights) when he detects darkness. In numerous parts of the game, you will go from light, dark, light, and to dark again while battling the baddies. The lighting is also real time, so guess what happens: your robo buddy will light up exactly according to the shadows, so he will blink, blink, blink, everytime he detects shadows - flawlessly! It just totally blew me away!
Also, it is bar none, the most violent N64 game out there, non-fighting! LOL I laughed myself to crap when I went into a room, and saw a baddy run around the corner. I heard him shooting. As soon as I turn, I saw him shooting at the furry creatures and they blew up into parts with blood all over!