Black Ops 1. The plot is interesting as well as the ending twist, even if they're a bit silly and predictable. I really dig the setting, weapons, missions and set-pieces, and the characters were at the very least memorable.
The only Modern Warfare i finished was 2 and i was extremely bored halfway-through, and can't remember a single thing about it aside from "gotta stop World War 3, bruh".
Black Ops 2 had some nice parts as well, mainly the ones set in the past. I feel like they kinda botched the plot by trying to tie the past and future timelines, which are set almost 40 years apart, leaving many, many plot holes. Ofc nobody plays COD for plot but i feel like BLOPS 1 did try a bit more.
That's it, those are the only CODs i ever played.