I'm not really the biggest fan of COD, I personally prefer battlefield games, however the original COD4, modern warfare, was extremely good. I haven't really enjoyed any COD game as much since that, although WW2 looks pretty good too.
the first black ops was my first ever call of duty experience. Actually one of the first, first person shooters i ever played. I think after BO2 however cod went severley downhill, the repetitiveness through the reuse of engines, story, etc became quite clear