What was Xbox's last killer app?

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Zippy6 said:

They've not really had a "killer app" since the 360 days. At least in terms of exclusives.

Yeah that's my thinking too, I genuinely can't think of anything on either Xbox One or Series that qualifies, which is crazy when you think about it, to go two generations without a killer app. Really puts into perspective why they've been struggling these past 13 years.