GoldenEye was a huge achievement and I love it but Perfect Dark is Perfect Dark. The single-player was a blast with fun levels, great music and super-silly characters. I really liked how you could exit the main menu and walk around in the HQ to access different parts of the game. Sure, it was less efficient but a nice detail nonetheless.
Multiplayer was incredible. It took the already fantastic multiplayer from GoldenEye and somehow made it even better. A real work of art.