I've owned all the COD games except BLOPS 2 - they get progressively worse from a multiplayer view.
COD4: Great maps, excellent game mechanics, and anyone can play it IF they tweak their settings.
And, the most important, dedicated servers.
WaW: Great concept, but sadly people just weren't as interested in a ww2 game. Zombies is the biggest thing it had going for it.
MW2: This is where the stupidity of no scoping, and broken knifing started. The knifing in this game worked like the beam sword in Halo. Stupid.
A lot of fun maps, but the design screamed of being a port from the console version. First game to use VAC (Valve Anti cheat) instead of Punk Buster. Worthless.
Black Ops: Lots of improvements here. Great game. The maps were good, but often had stupid glitches. Like bullet proof palm tree leaves, or little rocks you could get stuck on. Knifing wasn't QUITE so broken, but still pretty bad. Oh, and at least there was dedicated server support.
Also, uses VAC - so hackers can have a hey day if you don't have a dedicated server with an Admin.
MW3. I don't even know how to describe it. I feel like I'm playing a childrens game that requires almost no skill. There is no advantage to working the angles. Knowledge of maps makes minimul impact. Again, VAC only. So hackers get to have a party.
Some of you may differ from me on these opinions, but I find it very hard to recommend any of the COD games after 4.