I loved WaW and MW2, but the games have been going downhill since then.. Knifing in the Black Ops series is a joke. MW3's engine is nothing like its prequel, the maps are bad (ie. Dome), and the killstreaks are silly.
MW1 - 2nd Prestige
WaW -4th Prestige
MW2 - 10th Prestige
Black Ops I - 7th Prestige
MW3 - 2nd Prestige
Black Ops II - 2nd Prestige