Nintendo 64, Sega Saturn, Playstation 3, Xbox One; each the third console from their respective manufacturer, and each one made major mistakes that cost their company dearly.

Which of these do you feel was the biggest stumble, and why?

I could say Saturn because it sealed SEGA's fate. However, it was still a success in Japan and had an amazing library in Japan. PS3 bounced back. N64 was still a success for Nintendo.  Made them money and M64 and OoT alone were some of the most innovative and successful games of the era.

Xbox One. The reveal is just as bad if not worse than Saturnday E3 1995. (yes I know the reveal was not E3) Not only was MS reveal a disaster it came the same week Edward Snowden blew the whistle on the US Government spying on you. Meanwhile, MS says Kinect can see your heartbeat. If MS didn't have the deep pockets from its Windows division. XBO would have killed them entirely. 58 million is far more than 9 million, right? Yes, but the 1990s market was much smaller. 58 million in the modern market is not great and they only got that far because of MS deep pockets. Also, XBO had practically nothing for exclusives. What little it had mostly reviewed from mid to terrible. PS4 had it beat in 3rd party games and exclusives. The 3rd party games PS4/XBO shared base  PS4 often ran better. At least Saturn ran Marvel Vs Capcom and other 2D games better than PS1 or Grandia. I still maintain Saturn when counting the Japanese library is behind PS1 as the second-best console of that generation. Wii U sold far less than XBO but it had more exclusives worth playing. Saturn and Wii U have developed a cult status. XBO will never have a cult status. XBO marked the beginning of the end for the Xbox brand. It's seen with disdain or apathy now. XBO is one of the most meaningless consoles ever to exist that contributed nothing. Saturn did. N64 did. PS3 did. XBO had...*checks notes* a 100-dollar paperweight in the stupid Kinect no one wanted.

Xbox go home.

I forget the history with Sega during the 90s...from what I can recall, the Saturn was a powerful system (for its time) which had no mainline Sonic game (or no game of high quality). May not sound like a big deal by today's standards, though imagine if Mario was absent from NES/SNES/N64 (any one of these systems).
Since I'm more informed on XBOne, and relevancy bias has me still feeling the effects of XBOne's blunders (unlike Sega Saturn which knocked Sega out of the console industry exactly two years before I was born), I vote XBOne.

I voted for Sega Saturn but I can see why folks would sax Xbox One. I said Sega because it doesn't seem like they removed from that but Xbox never recovered either.

It's the Wii U. Wii was a huge success and Nintendo's best selling home console ever the Wii U was one of Nintendo's worst selling console. It's why Nintendo will never go back to traditional home console

Wii U was Nintendo's 6th console.

This is about each company's third console.

It was a bit more complicated then that. To briefly summarize some major issues:

• Sega was already on thin ice with customers after the underwhelming Mega CD and 32X.
• The Saturn was designed in 1992 and 1993 to be a high quality 2D console that was also capable of 3D graphics. This was largely wasted when gaming from 1995 onwards was largely defined by polygonal games.
• The Saturn's architecture was unusual, among other things having two CPU's. This along with other technical issues proved to be an early obstacle for making games on the console look as good as those on the PlayStation.
• Sega prioritized the Japanese market over the American or European markets, despite the Genesis doing better in the West than in Japan.
• The big new Sonic the Hedgehog game, Sonic X-treme, went through development hell and ended up being scrapped only a few months before its release date in 1996.
• The USA launch price was $100 higher than the PlayStation and said launch was premature, leading to hardware and software shortages.
• The biggest hit games on Saturn were largely ports of arcade games (Virtua Fighter 2, Daytona USA, Sega Rally, Virtua Cop, etc). Problem was, this was the generation where conversions of arcade games became increasingly overshadowed by games first made for consoles.

The Saturn DID have great games. Sega games like Virtua Fighter 2, Nights into Dreams, and Panzer Dragoon Saga were among the best-rated games of the era, and there were impressive 3rd party games like Tomb Raider (which was a Saturn exclusive for 6 weeks), Dead or Alive, Grandia, the best versions of Capcom's 2D fighting games, etc. But these weren't enough to save the platform outside of Japan.

My bad. I thought biggest stumble ever

Those are all pretty bad, but I've gotta say Sega Saturn. The Saturn is what really drove Sega out of the console market. (The damage had already been done by the time the Dreamcast released.) A lot of the Saturn's missteps had to do with hubris from Sega of Japan too. Sega of America wanted to keep supporting the Genesis, but Japan wanted to kill it off quicker, because Genesis/Mega Drive was a lot more successful in the US than Japan. Of course, the also botched the launch of the Saturn in the US and the library was not nearly as good as the Genesis library.

It's the console that drove a console maker to become 3rd party. What a disaster.

In terms of overall success, the Saturn. The darn thing sold 10% as well as the Genesis in North America and 13% as well as the Megadrive in Europe. Looking at total numbers makes it look like a bad decline, but even it being a relative success in Japan can obscure just how bad the decline was.

In terms of being a memorable console, the Xbox One. So far as I'm aware, the only major exclusives for the console are Halo 5, Forza 5, and Rare Replay. One of those is a retro compilation and the other two were not exactly fan favorites.