Doom is a unique game in that I'm assigning it a 9, even though it is 'technically' simple in design. The story isn't ground breaking, characters are meh, and it's pretty much just go to room A and kill everyone, then go to room B and kill everyone. So why is it special?
Doom succeeds fantastically because it does what it does with a top-tier level of grace. It's an absolute joy to play. Movement is smooth and satisfyingly free from frustration, guns feel great/badass, leveling up/upgrading happens at the perfect pace, and combat feels balanced (and satisfying; the instant kills are a delight). Although seemingly straightforward, few games can do all these in the manner of which Doom was able to achieve. The lack of regenerating health makes the game an actual challenge, but a very fair one. Again, few FPSs in recent times have been able to make me feel like when I died, it's was -completely- my own fault.
Satisfying and somewhat special, Doom was kind of a surprise; but a great, gory one