Maybe I'm just old fashioned but I don't think a Call of Duty game, or any first-person shooter set in World War 2, will ever top the first Call of Duty.
It has, in my opinion; more vast and better-constructed maps, more balanced gameplay and weapons, a more entertaining campaign and just a straight up better setting. I don't understand why people are seemingly leaving behind the World Wars as a setting for first person shooters.
Although, on the topic of more recent additions to the series, Modern Warfare is by no means a bad game. I never owned it, but I did enjoy playing the occasional split-screen game with friends. I still liked World at War more though.