Go with Conker here. It was raunchy, adult, gross, and hilarious. On a system produced by a company that many claimed and still claim is "kiddy", by a company that was mostly known for "kiddy" games, featuring characters last seen in the "kiddy" game Diddy Kong Racing, Conker's Bad fur Day defied all expectations by being a game that was unashamedly crude. It also happened to be an excellent platformer with a story that was a laugh riot. On top of that, it boasted the best graphics possible on the N64 and full voice-acting, on a cartridge!
Way ahead of its time. It showed that games CAN use rudeness and crudeness as both a gameplay element, a marketing tool, and to stand out from the crowd. It's an excellent game through-and-through and it also paved the way for another one of my favorite games, No More Heroes. It definitely deserves to be in the running.
Vote Conker!