Normally I would say Uncharted (I'm a big ND fan) but given how Uncharted 4's development was (departure of important figures 3/4 in the development of the game!)
Still the demos of the game have been very weel received so far and it (Uncharted 4) looks awesome, ND are at the top of their trade right now and I would be surprised if the game was less than stellar... which is something that cannot be said of Halo 5, the reception is good, but not amazing... To bring in more oil to the fire I will go as far as saying that the xbox one has no 'must have' exclusive, real console seller so far, nothing that scored above 90 in metacritics, something that is completely undeniably "great"... I think the only new AAA game that did this so far this gen is Bloodborne!obviously the next exclusive game expected to reach this kind of score is Uncharted 4, TR won't do this for sure...
Oh wait, Ori and the blind forest maybe? but this is on PC as well...