The Saturn. It was a total catastrophe. The high price and bungled launch killed it right out of the gate, and its games library was incredibly lackluster despite a handful of gems. There was no way Sega was going to recover from that. The Dreamcast was, sadly, dead on arrival. Say what you want about the other systems, but they didn't result in the death of a console brand like the Saturn did.
The Wii U? Yes, it was a failure sales-wise, but it still had quite a few good games, and deserved more attention than it got, and more importantly it didn't result in Nintendo going third-party.
The N64? Yes, Nintendo's decision to go with cartridges cost them big time, opening the door for PlayStation to become the dominant global brand it is, but it was still an excellent system with many of the best games of its generation. And at least Nintendo had the foresight to realize how important an analog stick was for 3D games.
The PS3? Yes, Sony lost a lot of market share, but it was still very popular (it actually won nearly everywhere outside of North America and the UK) and it had a lot of successful games, so it's not even a failure.
The Xbox One? Like Sony from last gen, MS did lose a lot of market share this generation, but the system is not a failure. It's still managed to be somewhat competitive for much of the generation in the U.S. and the UK (at least until the XBO hit a rough patch in 2017 that it never fully recovered from). In the U.S. it's managed to make it the closest generation since the 16-bit era. Also, the Xbox One X is an amazing system, and has been my primary since it came out.