For MP: CoD 2 < Cod:WaW < CoD4

What is Call of Duty MP without a Kar98? Not as good as it could be.

CoD4 was also the first game in the CoD series to introduce the perk system and silencers. And martyrdom. Teh horror. Takes less skills than ever to get kills in MP.

I kind of like the ranking system though; since it puts to display how much time you've spent - or wasted - playing the game online. To bad the PC version does not have the prestige mode.