Uncharted 4 then Halo.

Uncharted 4 > Halo 5 > Bloodborne

Either Uncharted 4 or Halo 5, nothing else will come close to them. We're looking at 7+ million sold lifetime. Quality and potential sequels may affect the outcome but I think Halo 5 will still be on top.

still halo 5.
but it will be a very close race with uncharted 4

Probably a PS4 game, like Uncharted or Bloodbourne, or Order 1886

I'll tell you what should, but won't be the best selling exclusive game. (It will have more depth and content than Watch Dogs, Titanfall, and Destiny combined minus dlc.)

Oh hell yeah. Game of the gen!

At the beginning of this year I would've said easily Halo. Now I'm fairly certain it will be some Playstation game.

Nothing SHOULD be the best-selling exclusive. While Xenoblade Chronicles X may become YOUR favourite game of the gen, it might not be mine. A PS4/XB1 or another Wii U exclusive might be better.

The tomorrow children

Edit: read it as besr exclusive , not best selling. 

Halo 5

Uncharted then Halo.
Unless................... Delays