Halo Reach
The last game made by Bungie, the original developer of the Halo series, this one feels like a true send off for the series.
The campaign is a prequel to all the other Halo games, but is best enjoyed after playing the other Bungie games. It follows a group of spartans after the Covenant, the aliens you fight throughout the series, begin to invade a planet called reach. Reach undoubtedly has the best storytelling of any game in the series and feels like it has real stakes. There are missions of all varieties seen in the previous games but perfected, and the weapon sandbox feels like it has the best variety. I would play this on heroic with skulls.
Firefight returns in this game and has a few improvements from Halo ODST, the multiplayer was also a lot of fun in this. You still have the issue of getting some people who are too good online so maybe play this one in private matches or splitscreen too. Like Halo 3, there is a good variety of custom maps and modes.
The Master Chief Collection is the best way to play this now.