Sorry I had to bring this up again but I think bar none COD 1 has the best single player. The Russian campaign by far the most memorable campaign of any of the series.
Gotta say that I definitely had the most fun playing through the CoD1 campaign way more than any of the others. CoD2 was OK, but I found it to be a little annoying at times. United Offensive wasn't as good as I'd hoped either.