MW2 was the first I played. I actually bought an Xbox after several years without gaming just for that game. Plus I had a bunch of friends that would always get on so it was a good time.
I LOVED the maps- Afghan, Storm, Favela, Highrise, Crash, Wasteland, Skidrow, Sub Base. The only one I can remember actually disliking was Karachi, although Derail was petty 'meh'.
Loved playing with the Tar-21. Shotguns as secondary weapon was much better as well; I used to go off with the Spas.
Black Ops was more arcade-y, if that's even the right word. Definite step down in graphics and sound effects from MW2 but fun nonetheless. I had a bunch of friends on this game as well so that kept it interesting.
The maps were decent. I loved Summit and that's about it. Didn't particularly hate any of them but none were memorable for me.
Killstreaks blew outside of Blackbird and Dogs (which were amazing, by the way).
It was probably the one that I was best at. I think I had like a 1.93 or something last time I played but felt like I was damn near unstoppable. That's due mostly to lightweight and marathon pro. I'd run around like a madman with an SMG and just annihilate everyone that wasn't camping. If everyone played run-n-gun like I did my k/d would have been much higher.
MW3 is pretty good, technically, but there's just something about it that doesn't click. I pretty much only play with one friend now and he's constantly bitching about it so maybe that's what it is.
Shottys are better than in Black Ops, so that's a plus.
The maps are kind of a unique situation for me. I pretty much hate every map; but I LOVE Seatown, Dome, Hardhat and Lockdown. I guess that kind of evens out to average but since I actually like some of the maps it gets the nod over Black Ops.
Plus this is the game I became proficient with the C4 and RPG. When I get hot with the explosives I'm a ****ing nightmare.
I have sooo many good memories making people explode!
To end this super-long-post-for-no-apparent-reason, I'd say MW2 was probably the best one.