C.O.D franchise on sale - which one has best single campaign?

[heartburnron]

I have Ghosts and have played MW3 on my boys XBOX. I fancy playing another one but I'm not in to multiplayer. The entire franchise is reduced on Steam just now - which one would you recommend for the single campaign?

 

[heartburnron]

Thanks for the replies guys. I'll check out cdkeys too before making any purchase.

 

[heartburnron]

Went for Black Ops 1 from cdkeys for £4.99. COD 4 was out of stock. Think I'll work my way through them all though. I used to find them quite boring but then up until recently I played everything the xbox controller - different experience playing with the mouse and keyboard...