If it's between the modern warfare series,Modern Warfare 3. World at War for the gore that this game only had in the series and Black Ops for the Wager Matches Stick and Stones rule.
I like mw2 personally as far as cod goes. I had no interest in playing any of the others, I bought black ops and I can tell you, I have no intention of playing it again haha.
It varies, they all have their pros and cons I.E. black ops- you can have a claymore and grenade MW3- dual attachments to assault rifle with tube. One has HC gungame the other only core etc. But for me it's MW3 all day everyday.