I would rank them with 1 being the hardest.
I didnt find COD 3 Hard at all,i got stuck at some points, but CODMW i found really hard,i left it for a bit then came back to it at a later date i couldnt do the "No fighting in the was room" level was really hard,also the "mile high" and WaW the shooting the 45 planes down took its dam time!!
COD 2 was only hard because of the respawing enemys!!