Call of Duty 4 is a great game that defines the FPS genre.

User Rating: 9 | Call of Duty 4: Modern Warfare PC
The surprising thing about COD4 isn't that it's innovative or that it has the best graphics out of any fps out there. Sure, it's got an addictive multiplayer that can absorb you for months, but at heart COD4 strings together the best elements of fps that there are to offer, such as the regenerating health system and varying weapons. The weapons in this game are very satisfying to shoot, the controls on the PC and Xbox (I have both) are intuitive, and the graphics make you feel like you're a real marine and SAS operative involved in a Russian conflict.
I think the one thing that holds Call of Duty 4 back is its story. Other reviewers will tell you that it is very intricate and has many surprising plot twists, but aside from a few witty remarks (namely by Cpt. Macmillian in the flashback level) I found the plot to be standard B- rated action flick material. I'm not saying that the story was bad, it just feels used.
In conclusion, this is a terrific game that lives up to any hype you may have heard (and there's a TON of it, especially since the game is over a year old now). If you have a PC that can handle it, then you want it for that platform because it obviously looks the best with high resolution AA graphics, but the console versions are no pushovers. My PC isn't top grade, as I have midrange 8 series graphics, but the game runs on high settings at a solid 50 fps with AA on. Buy this game!