yeah i played the call of duty 2 demo a few weeks ago and its realy good,Call of duty is also great for single player but i got bored of the multiplayer now.
Red Orchestra is pretty good. It's not always the best graphically, it uses the UT2k4 engine I think. There's always Battlefield1942/2/2142 but it is too arcadey for me.
i dont think its available for the pc i cant find it anywhere but call of duty 2 is out on the pc so i think number 3 should be :S my personal preference are the medal of honours any way but each to there own