I think that United Offensive was the best in the series, excluding COD4, because that's from a different time period. Although Call of Duty 2 was very intense and rightfully a classic in WWII gaming, United Offensive has the most diversity in its campaign and multiplayer gameplay, with nade cooking, sprinting, deployable machine guns, a variety of antitank weapons, vehicles, and simply intense gameplay. Call of Duty 4 is probably the best out of everything, but COD:UO gets my vote.
Here's my list of WWII era Call of Duty games.
1. United Offensive 2. COD 2 3. COD 2: Big Red One 4. COD 1 5. COD 3 6. COD: Finest Hour 7. COD: Roads to Victory
I haven't included COD5 because I haven't tried it out yet, but it will kick ass. I have waited a long time for it to bring the action to the Pacific. However, I'm a bit disappointed that they chose to include the Russians again instead of the British in Burma or the Chinese. Those are some truly noteworthy Pacific fighters.