Portal 2
The sequel to the game that redefined what FPS was capable of, Portal 2 really should already be in your library. Portal 2 takes the first game’s mechanics and cleverly adds to them, introducing spreadable fluids with various properties like reduced friction and massive “boinginess.”
It also takes you on a very (literally) deep voyage through the history of Aperture Science, the laboratory that created the portal gun and the omnipresent AI that administrates the facility, the infamous GLaDOS. While you don’t technically “shoot” anyone in Portal 2, it is in first-person and you have something called a gun. Plus it’s one of the best games ever, so it’s on the list.