For historical perspective developed in parallel with Half-Life 2, Medal of Honor: Allied Assault is worth a look. With Call of Duty being the revenue juggernaut it is now, I think we sometimes forget how it started. Allied Assault is a landmark game that’s an ancestor in CoD’s lineage.

On the other end of the timeline, the founders of the studio that developed the original Call of Duty went on to make Titanfall. Titanfall 2 has an excellent single-player campaign that holds up very well.