Call of duty 4!
I bought it yesterday and I've been playing it pretty much non-stop ever since.. I have completed it on easy level but there are unlockable things, an arcade mode etc. The arcade mode is excellent.
Also the dynamics of the game are great.
In Call of Duty 3, I always wondered why I couldn't shoot people through doors and walls, the physics were inconsistent, in the new game the physics are much better, you can shoot pots and plates, shoot pictures off the wall, shoot through wooden crates, tin sheetings, thin walls and kill enemies behind them.
It's really a step up from the last game.
Also some of the new styles of mission are fantasic, playing as a member of the SAS with night vision goggles is awesome, as is being a gunner in a hercules gunship at night looking throught thermal imaging equipment at the enemies and watching them die!
It's really like all those video clips that were on the 24 hour news channels during the Iraq war.
The only criticism is that the game is a little bit short but it is definately up there in my top 5 of all time.