Halo ODST

This one is just a campaign and a firefight mode. Getting it when it came out also gave you all the DLC maps for Halo 3 but they are all included in The Master Chief Collection now.

In the campaign, you play as an orbital drop ship trooper instead of a spartan like all the other games. This makes you a little weaker than the other games and pushes a bit more of a focus on stealth in the campaign. It is fun, and has an interesting non-linear narrative but doesn't quite reach the highs of Halo 3. I would play this on heroic.

Firefight is also a fun mode which has you survive against waves of enemies. Some people absolutely love it, I have had some fun with it but I much prefer the Halo campaigns.

The Master Chief Collection is the best way to play this now.