The Black Ops series (WaW, BO1, BO2) has more changes than the Modern Warfare series (MW1, MW2, MW3). For starters, the years in the Black Ops series are different (WaW: 1940's, BO1: 1960's, BO2: 1980's & 2025) while the Modern Warfare series are nearly fixed (MW1: 2011, MW2&3: 2016).
Xbox 360 Gamertag & Nintendo Network ID: TDPNeji Steam ID: NejiHyuga900
I have to say Cod4 put the series forward. WaW while I enjoyed it was stagnant and added nothing new. MW2 once again added tons of new things (although many broken and not well thought out). Black Ops basically refined everything MW2 did. MW3 was stagnant and added nothing to the series. BO2 continues to build off of the first one.