Hard to say with Uncharted vs Gears. I reckon with the fact that Gears:J tanked quite badly for a Gears game there is a very good chance Uncharted 4 will outsell Gears 5(?) in the USA.

Halo has no peer. But then, in the USA Halo sells mostly to hard core Halo fans from the Xbox days (Halo 2 NA total sales = 86.5% of Halo 3 NA total sales. Xbox NA sales = 32.7% of Xb 360 NA sales. This means while there was huge gen on gen growth for Xbox hardware, there was very little gen on gen growth for Halo. The vast majority of gen on gen growth for Halo came from outside NA.) So that only really guarantees an Xb one install base of about 16 million. For NA, I think people put too much stock in what Halo will do for Xb one. If Halo is going to do anything significant for Xb one it will be to get a lot of people outside of NA to buy Xb one. Short term I think Halo will have the bigger effect, but when considering the whole of the generation I don't think it will have the effect some people think. The lack of significant expansion of Halo in the 7th gen in the USA is essentially why most 360 owners beyond the core 16 million who bought Xbox are a swing market and Xb one needs something other than Halo (and Gears I'd wager) to get them to stick with Xbox. If most of the 360 buyers outside that core 16 million were former PS and PS2 owners it pretty much means they are fairly well disposed to going back to PS.