Day of Defeat, by a country mile.
The graphics are dated (but not terrible) and in terms of gameplay, it's way ahead of the rest.
Now if Valve could release Half-Life 2, and the DoD team port it over... and introduce iron-sighted shooting - it would be amazing.
Circle strafing is something you don't really see - and you can't jump up and down like a nutter (stamina is modelled).
Also, you can be a machine gun trooper and lug the MG-34 and MG-42 about; deployed in tripod mode you can give supressive fire for your comrades. You haven't seen carnage until you see the MG-42 in action on a narrow street.
The allies get the Browning .30 cal.