My fondest memory of Mario Kart is from the DS version and it involves the hated blue shell and a Wi-Fi match. I'm racing my best friend at Wario Stadium and he's got a pretty good lead going into the final stretch. I hit the last item box, get a blue shell, throw it up, and it hits him 2 feet before the finish line and I zip past him for the win. Really, it could be any one of dozens of great moments in multiplayer, but that's the one that sticks out most in my mind.