I would recommend L4D2. It really improves upon the original and I believe justifies its purchase price.
Some of the new modes of game play are very very fun. The survival mode is great each round lasts only as long as you can hold out (usually around 5 to 10 minutes). Survival acts as a great time killer while you are waiting for buddies to log on.
The versus mode is basically the best griefing engine ever made. Scavenging is fun too. There are tons of little achievements to get through out the 5 campaigns. Everything from holding out in a wrecked airplane vs hoards of zombies, carrying a garden gnome through 3 stages of one of the campaigns, trying to use only melee weapons through an entire campaign.
Its so entertaining my girlfriend will literally watch the finale events because they get so wild.
One person runs trying to distract the tank so that I can help my friend back to his feet. Then the tank goes down and hoards of zombies pour into the small stadium where we made our stand. You can see the helicopter, its less than 100 feet away! But, you can not shoot and reload fast enough to keep the zombies out of your path. With precious little life you desperately shoot and shove your way towards the chopper. Zombie hands grap and slow you, frantically you fire into the crowd! Dropping the shot gun and switching to a baseball bat you start wildly swinging. Your movement awkward and slow you barely make it onto the chopper, if your friends get knocked down there is no helping them, they are left... 4... dead!!! ahaha
EDIT:
L4D2 LAGS A BOATLOAD.
I dont know why multiplayer lags insanely.
But it shutters like no tomorrow. I think its the POS servers they are running.
But dude, i was comparing single player vs multi player to rule out hardware, and multi player is sometimes unplayable because of the amount of shutter you get.
That sucks, I don't know why that would happen, I haven't had any problems with it. There are lots of different servers though, maybe try again?