BO1 -- Consistent and empowering; best spawns, best-balanced explosives, and best of all you were in control of the pacing. The highest focus on gunplay and player-vs-player combat really expands what little skillgap there is in CoD (even though weapon design was sloppy). It was a delightful change from all the s*** in MW2.
CoD4 -- great, my second favorite, but people definitely have amnesia surrounding certain aspects of the game (mostly spawns and host advantage). No other CoD has maps so well suited for it's weapon and perk design.
W@W -- bleh. Absolute garabage connectivity for me at the time, and the the gameplay exacerbated it. Unscoped BAs were fun, but I didn't much care for anything else (including most maps and Zombies). The deathstroke was how DLC was handled.
MW2 -- dragged way way down by a mere handful of issues. People who say DC (let alone OMA tubes, or killstreaks, or deathstreaks, or spawns) wasn't a problem are either in denial or never played objective gametypes. Maps are mostly fine but painfully overrated. Half the "variety" is cheese. It's lasting legacy is an obsession and unwarranted sense of entitlement regarding powerful killstreaks. MW2 + Youtube poisoned the series.
MW3 -- awful lag/matchmaking, awful spawns, awful maps, awful game. The only positives are Specialist and the best Barebones gameplay in the series.
BO2 -- excellent on paper, but operationally messy and for some reason completely boring. Pick 10 is good but a bit overrated. Scorestreaks are great in theory but meh in execution. The biggest disappoint is the tremendous downgrade from BO1 in virtually all aspects of Dom.