Doom is, hands down, one of the best first person shooter games ever made, if not THE best FPS in history. Whilst it has questionable graphics in comparrison to modern games, it was still way ahead of it's time, as was the Quake series and offers so much more than any of these crappy Call of Duty games do these days.
The entire Doom series was my childhood in a nutshell, I will never forget when my Dad first installed it and let us play when we were only little kids. We had so many hours of fun and so many different levels and difficulty settings to try and complete.
To sum it up, Doom, Doom II and Final Doom are some of the best games that were ever produced. Seriously, why WOULDN'T you want the whole Doom series?