My vote lies solidly with CoD4. MW2 was broken so very, very badly - I managed a 2.0+ career k/d with a minimum of effort (Predator+Harrier+AC130, heyoooo), and I played a lot of objective-based games where I was the only one actually trying to plant the bomb / cap the point / what-have-you. CoD4, on the other hand, requires a much slower, more tactical style of play, especially if you give a shit about winning objective-based games.
Honestly, though, all that really matters in any game that doesn't completely suck (looking at you, Gears 2; spam grenades and two-piece, you'll win every time) is situational awareness. I'm not talking about using a headset, either; that's just having another tool at your disposal, and I manage quite well without it. I'm talking about knowing the maps and learning likely actions of other players, then manipulating those actions with covering fire, various grenades, and any other tools at your disposal so as to effectively and efficiently eliminate your enemy (alliteration yay). The various Wolfensteins, UTs, CSs, TFs, Dooms, Quakes, Gears 1, all CoDs but MW2 and 2:BRO (netcode durrrr), and both BF:BCs all require this skill, and if you're good enough at keeping your shit together to do well in any of these, then with a little practice you can excel in any of the others.
...finally, in my admittedly limited experience, MoH 2010 is a campy shitfest. Just sayin'.