The video game series Dead Space by Visceral Games has broken the collective zombie mold. Even when other video game series such as Resident Evil or Left 4 Dead use more traditional zombies as the primary enemy. Traditional zombies are usually dimwitted cannon fodder that infect the living through bites or scratches who's only real advantage is sheer numbers. Necromorphs are unusual in the fact that they don't infect people by biting them and that they are incredibly intelligent recognizing when an individual has a weapon.
From what has been established in the Dead Space canon, is that the necromorphs are spawned by infected corpses that are mutated and repurposed into various grotesque shapes that either resemble humans or don't resemble people at all. They adapt environments rather quickly and are very violent, what is known is that the necrotic flesh will reanimate in the presence of a Marker. A Marker or at least the original one was found here on Earth and was deemed to be of true alien origin. The effects of the Marker cause individuals to become hyper-psychotic and delusional. People under the influence of the Marker have visions of dead relatives or become paranoid and in their paranoia kill themselves or others. In the Dead Space universe a religion has been established around the worship of the Markers. What the believers of the religion don't know is the true effects of the Marker and what it does to people. Often believers will be especially susceptible to the Marker's influence because of their gullibility. Once they become psychotic they will kill themselves and leave corpses for necromorphs to infect. Necromorphs only have one drive and that is to kill and infect more people to produce more necromorphs.
Another area where necromorphs break with tradition is that they cannot be killed by shots to the head. In other zombie movies, books, tv show, etc, head shots or shots that sever the spinal column always result in permenant death. A necromorph that has had its head removed will continue to attack its prey until killed. The only way to do this is by completely dismembering them, often through the removal of limbs. Because the series takes place in the future this is done through the usage of modified mining equipment such as a Plasma Cutter or Line Gun. Traditional firearms such as pistols, rifles and shotguns will not do enough damage to kill a necromorph. Often if firearms are used it is to blow the limbs off of necromorphs, but the instinct to shoot center mass often leads individuals to be killed by the necromorphs. While it might seem enough to simply blow holes in a necromorph to kill them, it often leads to that individuals death.
Another thing that sets them apart is that Necromorphs are highly intelligent and will use the environment they have adapted to in order to set up ambushes for unsuspecting people. Such types of necromorphs like the Stalker or the Pack will use the environment to kill their prey. Even if a necromorph is alone it will recognize that a person has a weapon and instead of rushing at the individual like other cannon fodder zombies do, a necromorph will hop into a vent or air duct and use it to get around the individual usually coming out from the side or behind a person. Given all of these factors I think necromorphs are the ultimate zombie because they are very hard to kill, they are intelligent and highly adaptable to any environment they encounter whether the narrow passageways of a deep space mining vessel to the vacuum of space they always find a way to kill their target.