Halo 3
This is a great game. The story mode is really good. Halo 3 is actually really cool because the developers were some of the first pioneers to implement AI in enemy soldiers. They were more intelligent, and it made the game feel more organic and challenging. I really enjoyed the story too.
Halo 5: Guardians is also a game I really loved. I basically only played the multiplayer, but it was awesome. I really liked big team battle in this game. It was just fun to have a huge map and have access to all the different weapons and vehicles.
All of the Halo games are available on the Xbox store and are free if you have the Xbox Game pass.