Really, the Saturn takes the cake on this hands down. It sealed the fate of Sega's hardware and allowed its competition to dominate handily. The real question is what's second.
XBOne has damaged the XBox brand in a similar way, but it hasn't threatened to derail the entirety of MS Gaming (MS gaming is too big for XBox's failure to really impact it).
The PS3 was a money sinkhole that burnt all previous PS profits and nearly bankrupt Sony, but afterwards the PS4 kept its dominance and they recovered their losses for the most part.
The N64's high price and cartriges allowed the PS to thrive, but Nintendo was very safe with the GameBoy's dominance, so it was really in no danger at all (a situation that continued with the GC-GBA).