Greatest PC game ever. Believe it.
This is the one that started it all. Well, not technically, cause id software actually started the First Person Shooter genre with their nearly as good Wolfenstein 3D. But uh, this was the big one. The one that truly changed the landscape and broke the boundaries. Taking the initial gameplay of W3D, id elevated it to a new level, literally, with elevated level design and way more colorful backgrounds. From a ravaged Mars base to the bowels of Hell itself, Doom was an exhilerating experience back in the good ol days of 1994. Nothing like it had ever been seen before, and pumping a shotgun shell into a demonic hellspawn for the first time was a truly empowering moment.
Has the first person shooter genre moved beyond the corridor crawls of Doom and its many clones? Well... yes, it's number one for nostalgic reasons, but history must be respected. They're the foundation for what we have now, and should be appreciated. There aren't enough adjectives to describe everything Doom made me feel when I played it, but this landmark in computer gaming is one four letter word that cannot be tossed around lightly.