Left 4 Dead is my favorite zombie game. Not only is it fun, it kind of eases you into horror based games. For beginners (someone who just started playing these types of games) this game is perfect for them. Usually, they are hesitant to play games like RE or Silent Hill, especially if they are used to playing a whole different type of genre. But because you have people to help you, it's not so scary. Did I mention that it was fun? It's a great bonding experience as well, because you and three other friends can play at the same time (online ow=r with a system link). It's also a somewhat scary. Only when a hunter pops out of NOWHERE, when there is a witch, or anytime there is a tank and don't lie and say you don't freak out whenever you are cornered by a tank or yell "Lights off!" to your friends in a panicked voice whenever there is a witch. Technically, they are infected, but it has the same feel as a zombie game.
Left 4 Dead for believable zombies and fun xbox live gameplay (but otherwise you shouldn't get it unless you have xbox live or just get the PC version), RE: 4 for scary gameplay and pure AWESOME zombies with a great backstory, and then of course there's dead space.
i am about to get all nerdy on y'all... the newer resident evil games does not consist of zombies but humans infected with the t virus... so its not a zombie survival game but it's its' own genre... haha...