My favourite game ever for a long time, only competing with Dark Souls.
It has everything that made the original Doom great but with a new weapon (the double barreled shotgun, the greatest invention of mankind) new enemies, and new levels based on innovative themes, like the city levels (even if they're not my favourite type of levels, i appreciate the variety).
There's not much to say about this game, it's just great and paved the way for many actual games. Must play.

Oh, and the music is great.