To be honest mate (multiplayer aside) I have never felt let down by a COD title. I remember feeling the latter MOH games being intrinsically scripted to the point of almost being on auto pilot, which is the cardinal sin as far as I'm concerned. Somewhere between FarCry 2 and MOH is about right for me. Constrained freedom rather than a total sandbox approach, although the time I am able to set aside for gaming probably helps make that descision for me. Those with more time will likely see it slightly differently.
And I don't really have a leaning toward Infinty Ward or Treyarch. Campaign wise I loved World at War. Especially assaulting the steps of the buildings in the penultimate mission. I'm very easily satisfied, give me a gun (don't care what sort), good cinematics and a beautifully balanced control feel and I'm happy. I don't want tired, heavy, "simulated" movement (Killzone for example). I hate realistic movement. If I wanted realism I'd join the TA.
Twitch shooting all the way for me.