Conker's Bad Fur Day.
I was too young (and too innocent) to own such a game... but every time I went over to my friends house, we'd play that game 'til kingdom come. And what a game that was. I've never laughed so hard (The Great Mighty Poo), been so scared (The Spooky Chapter), or been so infatuated (the Tickly Bees...) in a video game world before. And the multiplayer was fantastic as well (Chainsaw and Magnum ftw).