We could certainly look back years from now and say Xbox One depending on how things play out. Sega Saturn is probably the right answer since it lead to their demise, but I'm going to go with the N64. Nintendo going with cartridge lost them every single rpg they had as Square, Enix, Capcom, etc all left Nintendo for Sony. I had a great time with the N64 as Nintendo, Rare, and Acclaim were firing on all cylinders this generation. If not for the Wii, and Nintendo's handheld systems the N64 and the resulting Gamecube might have ended them.