Easily Phil Spencer. After all, he was there all along with Don Mattrick to conceive the Xbox One blunder. Mattrick had to take the blame and got fired, but it was always a bit weird that some other higher-up from this team of leadership took over and was supposed to represent a humbled Xbox team that had a better understanding of what gamers want.

It was us, the audience

You mean the people who bought an Xbox or just every gamer in general?

Is there concrete evidence for how much of the BS is Mattrick or Phil's fault vs Nadella or Ballmer? Discounting that huge aspect I'd have to go with Phil since while he did do a good job salvaging the Xbox One at first the amount of poor management of Xbox studios in the past decade has been wild. As mentioned earlier the Xbox One lineup in its first couple years had a good argument of being better than what the PS4 had at the time but 2016 onward it's been no contest the entire time until recently other than maybe 2021.

In terms of Xbox as a whole I think Phil has done well since Xbox is a truly massive publisher now making more money than ever and GP is a good option for a lot of people and whatnot but in terms of the traditional console side of things that part is gonna basically die under him so while it got significantly wounded towards the end of Mattrick's time it's reached death's door under Phil so it has to be him for that.

Add Satya Nadella to the poll but definitely Mattrick.

Phil Spence was General Manager of Microsoft Game Studios all along while Don Mattrick fiddled with X360 and XBox1 development. PS was sitting at the Boss table when every and all decisions were waved through by the people at the table. It would have been his job and duty to stop the tvtvsportstvtv madness and put games to the front line as this was his job. Apparently he didn't and the rest is history.

Don Mattrick. No doubts. Without the Xbox One debacle, Xbox would not be that far behind in the console area.
Now we can argue that Xbox (in general) is not in trouble at all right now. Only people looking at the "console" would see it that way.

One thing that people keeps ignoring is that after having a market at more than 70% for Sony with their PlayStation Four, most people will not switch after buying all their games in for the Sony ecosystem, and it was, from the start an almost impossible hill to climb back; most people would prefer to go to the next Sony console NO MATTER what and then pretend it is for whatever reason they want to make up, but the reason was their wallet and not having to buy all those games again and keeping their trophies etc, simple as that... 

People may pretend that it is not true if they want, reality is that Xbox this generation started with an almost impossible task and seeing where they are today is definitely a success in my opinion.

I'm looking at this from a business perspective and Xbox is definitely in a better place than when the generation started. People needs to stop freaking out when an Xbox studio game come out on other platform, or when studios are closed, or games are cancelled. This happens every year and not only with Microsoft; but for some reason people are making big fuzz about it when it is Microsoft... It sucks for the people losing their jobs but it is just how businesses with shareholders works including Sony. 

Like what? Being able to use the library they spent so much money while buying into the Play Station ecosystem on any new Xbox console? Cause that's definitely 90% of the issue with this generation. Xbox could never have come back from the Xbox One debacle after that. 70% of the market was locked down, making people switch back to Xbox was almost impossible after that and Mattrick is 100% responsible for that as HE was the one leading the show, not Spencer.

Not to mention, Mattrick left by himself to go screw up another good company after that, he did not get fired (only some people on the internet speculated that...)

On the subject of the Xbox One fiasco, Phil Spencer said that he was part of the decision making for it, and can't act as if he had nothing to do with it. Not sure if he impacted it in any meaningful way though. If he did, then maybe he would be the answer.

But Xbox current situation may be a consequence of what happened with Xbox One's launch where they never recovered the position they had with 360 and felt forced to go in a different direction rather than relying on their console.

This is debatable to say the least. Not a debate that I want to have though.