Are you ready for some Halo on the Xbox One? Microsoft just added four Halo games to the system's backward compatibility library. This means Halo: Combat Evolved Anniversary, Halo 3, Halo 3: ODST, and Halo 4 are all available to play on the Xbox One today.
Backwards compatibility for these games were teased earlier this year and all four Halo games are actually already playable on the Xbox One in some form or another. Halo Anniversary, Halo 3, and Halo 4 are all part of the Halo Master Chief Collection (which also includes Halo 2 Anniversary). Halo 3: ODST was alo remastered for the Xbox One and Halo: Reach has been backwards compatible for some time.
Incidentally, the reason why Halo 2 Anniversary isn't backwards compatible is because the game never actually came out for the Xbox 360. The first Halo 2 released on the original Xbox and later re-emerged on the Xbox One as part of the Master Chief Collection.
Luckily Microsoft is adding support for original Xbox games later this year according to the company's announcement at this year's E3. So hopefully you can get the original version of Halo 2 on your Xbox One that way if you're so inclined.
Oh and if you're wondering, the answer is Halo 3:ODST. That's the best Halo game along with Halo: Reach.