As far as exclusives:

Halo 5
Gears 4
Forza 5

These games would help sale Explode.(i would be one)

Jade Empire 2 (Xbox Exclusive)
Killer Instinct (remake)
Halo 5
Splinter Cell
Gears Of War :The Begining
Ninety-Nine Nights 3
Project Gotham Racing :Velocity

I think Microsoft is missing out on the biggest one.

Halo4, starting the new trilogy with the new team new art direction exclusively on new hardware. Halo5 just won't have the same sales impact as 4 could potentially have had.

Lets look at this from three perspectives. 3 over all titles another with 3 first party titles and the last with three third party titles.

Overall I would say the best imaginable Nextbox launch line up for sales.
1.GTA6 (Exclusively on Nextbox)
2.Halo5 (Remake of Halo 2 included)
3.CallOfDuty:ModernWarFare4 (Exclusively on Nextbox)

The best possible third party release list is
1.GTA6 exclusive
2.CallOfDuty:ModernWarfare4 (Exclusive)
3.BattleField4 (Exclusive)

The best possible first party three titles
Halo5 (Including remake of Halo2)
GearsOfWar4
Project Forza (A combo game that takes the best aspects of both franchises mixes them with some other game play elements bringing in some veteran developers from the old Project Gotham team to help shore it up)

Of course the list of three included the three strongest properties on the Xbox. I did leave out some others.

A new Fable would kill. This time however Fable has a campaign size similar to Skyrim of 300 hours and tons more customizability. The ability to have other players up to 16 play cooperative with you on certain missions like escorts and attacks on enemy bases giving it Live features the series so desperately lacks.

A New Kinect Adventures, A no brained. However despite the over whelming success the first entry had on Kinect. Repeating that success is not only not guaranteed but a true risky test of an IP. Their are several one hit wonders in the industry and their is no guarantee the title could hold a new platform like Nextbox.

Of course I am betting that new IP might be able to do just as good as any of the three's I listed. If Microsoft hired Epic to create a new IP using Unreal4 and the talented team that did Gears their is no telling what they could do. Same could be said about Rockstar or Crytek or even the likes of UbiSoft Montreal. Their is no telling what the best minds in gaming are capable of.

Lastly the lists I provided are the best imaginable lineups. Exclusivity is vital to their ability to sell Nextbox like gang busters in its first year. However exclusivity would likely cost Microsoft 200+ mill each and that kind of cost seems illogical considering the economic situation globally. If the titles are not exclusive then they will not benefit Microsoft all to much as they will be on the competitors hardware as well. Microsoft did pay 50-million for time exclusive DLC for GTA4 so 200+ mill for a GTA exclusive on Nextbox doesn't sound nearly as far fetched that is of course depending on how much money Rockstar would demand and how long the game would remain exclusive.

Lastly the three titles all exclusively on Nextbox would be such drastic over kill its not funny. Microsoft would need fifteen or so million hardware units on store shelves during the launch window to meet demand assuming their is a healthy lineup supporting the three. If Microsoft did such a massive move I would say that it would be the absolute best way to catch up to Nintendo in as short a time as possible if not enable them to surpass Nintendo's lead within a year of being on shelves! It would be the most drastic move Microsoft has ever made!

Shenmue 3
Skies of Arcadia 2
Panzer Dragon Saga 2
Streets of Rage XBL Remix

Next Xbox - Dreamcast 2! Do it Microsoft!

Left 4 Dead 3

Halo Wars Kinect Control

Forza

halo is definitely one

i would say gears is another maybe even forza and for a third i would want to see a new IP with adventure and action elements

Yeah I did take some liberties with AW 'cause I like it :P

Are you going to do this for all 3 of them?

For absolute best sales microsoft will need to buy Activision, Nintendo, Square, EA, and Rockstar.

Launch titles all exclusive to Nextbox

Call of Duty 

Grand Theft Auto 5

Fifa 2014

Madden 2014

Halo 5

New Super Mario Bros Kinect

Mario Kart Kinect

Every single future Final Fantasy and Dragon Quest game

NintendoPie said: Are you going to do this for all 3 of them?

Not for Wii U, but you can do that one if you like as long as you reference me somewhere in the OP.