Yeah I think I agree that while the Kinect device itself was a killer app, Kinect Adventures itself was kind of incidental and not a killer app in and of itself. If say Kinect Sports or Kinectimals had been bundled they would have sold just as well off the back of the hardware's novelty factor.

Forza Horizon 6 is a killer app. Anyone that thinks otherwise is any idiot. Fable should be its next killer app.

Xbox has had a bunch of great games since its glory days, but not many that have single-handedly driven widespread console adoption.

Maybe the one-two punch of Forza Horizon 5 and Halo Infinite in November/December 2021? I recall there was a huge spike in hardware sales around that time.

Halo Infinite was not a killer app.... the reveal was so bad they had to backtrack, and give it another year in the oven.
When it launched, it fizzled out quite quick.  The bad pr of the reveal, I would say killed its status as such. 
At launch I think it had like a 86 metacritic score (today it sits at 87), mostly for its multiplayer aspect.

In June 2026.... Halo Infinate is ranked 49th on xbox's "most played list".
https://www.windowscentral.com/gaming/halo/halo-infinite-is-slipping-down-xbox-most-played-but-master-chief-collection-refuses-to-be-fragged

It has been almost 5years.... so yeah that makes sense.
However its getting beaten by older Halo Titles in the "most played list".

Just doesn't seem like a killer app, the type op was speaking off.

Yeah, you're right. I was trying to find some modern example that might fit the bill, and even then I had to include two different games across a limited two-month period.

But I do think Horizon 5 and Infinite moved the needle to some degree. In the US, in December 2021, Infinite debuted as the best-selling game on Xbox platforms, and the second best-selling game across the board (it lost to Call of Duty: Vanguard). The same month, Xbox Series outsold PS5 in both the US and the UK. And in January 2022, Microsoft reported that 18 million people had played Forza Horizon 5 and more than 20 million had played Halo Infinite.

Of course, there were stock issues everywhere at the time. And it's difficult to determine the exact influence of these games on Xbox Series, since they launched on PC as well.

Yeah I never got the impression that Halo Infinite was a killer app tbh, by then the IP seemed to have lost most of its sway, and the game was dogged by negativity since its infamously underwhelming reveal.

Since no X Box console sold over 100 million units any game that sold over a million copies can be considered a system seller. Also, since over 75 percent of people buy Playstation and X Box consoles to mainly play 3rd party games they actually share a lot of the same system seller games.

Forza Horizon 6, but it mainly sells on PC and will be ported to PS5 soon.

Blue Dragon

Xbox Game Pass. It's arguably the biggest reason people have bought the Xbox Series X/S. Just compare Gamepass's Subscriber count to the total number of Xbox's sold this gen (30 million vs 35 million).

Besides that, there have been a few games post-2010 that have also moved hardware:

• Forza Horizon series (2012 onwards) - FH3 passed 10 million players by 2018, FH4 had 24 million players by 2020, and FH5 had 20 million players within seven months. Even if we assume half of these are on PC, that's still a significant exclusive.
• Grand Theft Auto V (2022) - No really, if you look at the weekly sales of the Xbox Series, they shot up about 80% the week GTA5 got its XS version in May 2023. That technically counts.
• Minecraft: Xbox 360 Version (2012) - VGChartz has the X360 version of Minecraft alone selling 13 million units. This was due in part to the X360 version getting released just a few months after the main PC release, with a retail release in 2013. Other consoles had to wait more than a year after that to get their versions. Minecraft bundles also seem to have been widespread at the end of the 360's life.
• PUBG: Battlegrounds - Like with Minecraft, PUBG was one of the world's biggest games, and the Xbox 1 was the first console you could play it on. 9 million people bought it on Xbox 1 before the PS4 version came out.

There are also a few dishonorable mentions I could make (Starfield, Halo 5/Infinite, Rise of the Tomb Raider, etc). And arguably Sea of Thieves, though most of its sales seem to have come after they went multiplatform.