Left 4 Dead 2!

I went over to a friend's house and played some COD :MW2 and mostly L4D2. I found the online play for both of them fun and amusing, but more in L4D2 (when I play with people who know what they're doing.) Some modes in L4D are fun and interesting, like being able to play as the Infected (zombies), but in order to be effected when infected -- organization within the team is REQUIRED. Otherwise, since the zombies die so easy, the infected team will just "fail." On the other hand, when playing as the Survivors (humans), not as much organization is needed, but communication is still necessary.

It is extremely fun to play with those who care about their teammates and know what they are doing. When you are down (knocked down by a zombie), the other team members are nice enough, or care enough, to come back and get you. When playing with those who don't know where to go/what to do, they're mostly busy with shooting zombies until the their entire screen is cleared (which can never happen, as zombies spawn at a constant rate). As such... they don't come get you until near death.

Overall, I think L4D2 is really fun, especially when playing with friends -- since you know that they will come back for you if you need help.