If we’re measuring only by the explosiveness of the collapse, it’s Sega. But, honestly, I find Xbox’s fall from grace far more embarrassing, considering Microsoft’s nearly unlimited resources.

Sega made a lot of bad business decisions that ended up hurting the brand. Meanwhile, hurting the brand IS the business decision at Microsoft. I think that’s the key difference.

In that case Xbox is Nickelback. 

While I would agree that their decisions with the 32X and Saturn were catastrophically bad, I would argue that Microsoft's E3 2013 or their lack of killer apps for like two generations now were in the same ballpark of terrible.

Apt reply.

Atari fucked up so bad they had to bury games in a desert. They launched a console with broken controllers out of the box and no BC with 5200. They made their CD add on for Jaguar look like a toilet. Atari sold the mold of Jaguar to dentists and in the 90s dental equipment that look like Jaguars were in offices. Atari ran a contest giving out real jewel and gold goblets and such but ran out of money and never gave the sword away. 

ATARI TURNED DOWN BUYING A 30% OF APPLE FOR ONLY 50K! Atari is the biggest crash and burn ever and few realize it.

ATM... SEGA.

SEGA consoles are dead.
Xbox consoles are on Life support.

Life support > Dead.

SEGA, the company that isnt investing on their own platform (digital or physical).

Even if Xbox as a physical platform dies, Microsoft Gaming is something that will likely stay on to be something.

Yes.  Great comp.

Still around and making millions?  

Sega. They've had only one successful console (Genesis/Mega Drive). Because of the Saturn they lost 3rd party support for the Dreamcast. The 32x and Saturn lost them customers in the West. They lost so much money that if it wasn't for an investor forgiving Sega's debt to him they most likely wouldn't exist today.
Xbox consoles are still being sold and have great 3rd party support. We know there will be another Xbox system, so you cannot say the consoles are dead yet.