Black Ops 2 feels the smoothest online. doesnt feel like your weighted down and slow.
Black Ops 1 had that unique feel to it, from the Vietnam time period.
World at War was a return to 1944 and WWII, sadly its been fukked over by hackers
MW3 actually made it feel like you were playing as a team.
MW2 is what started that fakkit ass quick scope bullshyt
the GOATest is Call of Duty 2: Big Red One... it started the whole Score Streaks thing (Assualt got binoculars after 5 kills that allowed to have an artillery bombardment and shyt)