BF2 is what MW2 should be on the PC for multiplayer.
CS:S is still the benchmark for the best online experience and the wealth of mods and maps is staggering. In the time it takes my PS3 to arrange one MW2 match, I can have been fragged five or six times in CS:S - very fast, very fun, very little downtime.
I enjoyed MW2 on the PS3 in single-player, but it is pretty naff compared to HL2 / Episode 1 & 2. Pad-control is just useless after the speed and accuracy of a mouse and auto-aim is for wimps
Forgot to add I've only played L4D and not the sequel and never played online. On the PC that has to be the most frantic, 'holy crap!' moments ever devised - nothing like as technical as CS:S or BF2, but hugely entertaining - I could imagine having 2 beers playing CS:S and then a few (several) more on L4D once my reactions have been too dulled by alcohol
I would have to say COD4 is probably still the best but it does lack a weapon selection. I must say BC2 made tubes not as noobish like they are in COD4 which is nice. They both are enjoyable and my gaming time is becoming split between them now days. I do play LFD every now and then cause the socializing is fun sometimes. Have not tried LFD2 yet. I might get it when the price comes down, that is why i bought LFD cause it was only 20 bucks.
I like L4D2 versus. Playing as infected makes an interesting change from the usual FPS where you have a bunch of weapons which are - broadly speaking - all projectile based and are used to kill people. As the infected you really do have to work as a team to exploit the individual attacks of the infected and use them in such a co-ordinated was as to separate or incapactitate all the survivors at once.
BC2 is looking really nice. Final judgment when the retail comes out with the other game modes and maps, and see how the balance and squadplay elements pan out.