At the moment I am really enjoying the PC Left 4 Dead 2. I got it today on steam since it was suddenly free, now that is an offer worth not missing!
I have been playing a lot of it today and I have to say it much better than the Xbox 360 and PS3 ports. Of course the controls are not quite as good since I don't have a controller for my new PC yet but I am really enjoying the multiplayer anyway.

The only game that tops this is the amazing FPS Doom. Now I am talking about this second because it doesn't quite count as a Zombie slayer but the majority of the enemies you will be facing are Zombies or relatable to Zombies. I still play that game to date and is by far my favourite FPS, even past my beloved Halo 3 and 4.

GenesisJunkie : I love that game, it is a shame it was under shadowed by the big hits on both the SNES and Gen. To be honest I prefer the SNES version, just because of the controller. The games are close otherwise but it just feels smoother on a SNES.

I've played Dead Nation, Left 4 Dead 2 PC, Resident Evil 6, and Dead Space 3. Out of all the games Dead Space 3 was the better zombie game mostly because it's challenging. The Zombies are quick and they would come out in all directions mostly from behind and in dark areas which ends up killing me a lot of the time.

I have to go with Left 4 Dead 1 & 2. It was the game that got me back into video games. DayZ comes close, I have yet to play the stand alone, so who knows. It may become my new favorite. 

I'm going with Half-Life. It's not Zombie based, I don't think, but those zombies are incredibly creepy. 

I haven't played L4D or RE, yet. I hope to play them soon now that I see they're good zombie games. In second right now would be Plants v Zombies =P

I would have to say The Walking Dead is my favorite Zombie game! I still haven't played the new parts that have came out yet to it.. I have only got a chance to play the first game that they came out with. I really want the new episodes of it, I am so excited!

um, I really don't know XD I am not a really big fan of zombie games, but I will say Call OF Duty Zombies? Maybe plants V Zombies.

I would have to say Left 4 dead 2. I actually don't think this game is as good as everyone says, but i still love it.

I'll have to say Left 4 Dead 2... Well, yeah, I've only played the demo, but it tops every other zombie game. Ever. It's immensely fun playing split screen mode, even on the demo (which only has 2 levels). I've played the demo a total of three times and beaten it twice (last time we got interrupted by dinner) and I still don't get tired of it.

Another good zombie game is Zombies Ate My Neighbors which is for both Sega Genesis and the Super Nintendo. Of course, the Genesis version is better because the Sega Genesis is always better.

I'm gonna say Dead Rising 2 because the huge array of weapons that you can use and combine is staggering, and you can customize Chuck. The game also has a serious plot, but is very humorous-and I like that.

I would say Left 4 Dead 2 because of the realism, the large number of weapons, and the gameplay.
The game supports co-op, so you and your friends are able to play the singleplayer campaign but, of course, it will be much harder and cooler as well.