Resident Evil 4
It's one of very few games that I can find to fault with. Every single thing about it is amazing, from its white-knuckle action to its desolate atmosphere to its pitch perfect pacing, and it all gels together to create an experience like no other.
It effortlessly manages to top itself over and over, building from one thrilling encounter to the next like a symphony of adrenaline.
It's not just the finest game of the 2000s, but one of the greatest games of all time.