Doom should be understood not as a "first-person shooter" but as a highly asymmetrical real-time chess variant. While the first Doom is arguably the better experience right out of the box, mainly due to the sequel's less consistent level design, the addition of the super shotgun and a richer bestiary add a great deal of depth to the game. Of course, Doom II is not just the game out of the proverbial box: it's also the entry ticket to perhaps the single richest archive of fan-made levels for any game, and it's this fact that makes it an essential game. Many people tend to regard fan-made content as necessarily inferior to "professional" design work, especially when it dares to push the difficulty ceiling beyond their comfort zone, but this is ignorance and laziness. The truth is quite the opposite: the collective effort of iteration over years and decades inevitably pushes a game's level design in directions that no single team could accomplish or even imagine in the span of a release cycle or two. Of course that isn't possible if the base game isn't good at its core. Doom II just happens to be perfect.