I voted slow.
While I don't hate fast ones (I love 28 Days Later!), I feel like the slow ones are actually more menacing.
Like Maddness666 said, they represent a sense of inevitability. That death is slowly coming for you, it's not going to stop, and the longer you wait to do anything about it, the more futile escape is going to be. Add on top of that the fact that slow zombies are typically shown as being incredibly durable to weapon damage that doesn't directly destroy the brain. Though the game series is incredibly campy, I always liked the moment in the introductory cutscene to Resident Evil 3 where you see the Raccoon City Police Department and S.W.A.T. deploy themselves and their vehicles as to create a wall across the road and an effective firing line. The police open fire on the wave of approaching slow zombies with handguns, shotguns, and submachine guns doing very little visible damage to the approaching zombies. Despite their best efforts, the police are completely overrun, one of the last moments of the cutscene being the reflection on a bloody S.W.A.T. helmet visor of a zombie shambling through the police barricade. To me, THAT is what slow zombies are all about, and what makes them (in my opinion), more threatening than the fast-but-incredibly-fragile variant.
While I don't hate fast ones (I love 28 Days Later!), I feel like the slow ones are actually more menacing.
Like Maddness666 said, they represent a sense of inevitability. That death is slowly coming for you, it's not going to stop, and the longer you wait to do anything about it, the more futile escape is going to be. Add on top of that the fact that slow zombies are typically shown as being incredibly durable to weapon damage that doesn't directly destroy the brain. Though the game series is incredibly campy, I always liked the moment in the introductory cutscene to Resident Evil 3 where you see the Raccoon City Police Department and S.W.A.T. deploy themselves and their vehicles as to create a wall across the road and an effective firing line. The police open fire on the wave of approaching slow zombies with handguns, shotguns, and submachine guns doing very little visible damage to the approaching zombies. Despite their best efforts, the police are completely overrun, one of the last moments of the cutscene being the reflection on a bloody S.W.A.T. helmet visor of a zombie shambling through the police barricade. To me, THAT is what slow zombies are all about, and what makes them (in my opinion), more threatening than the fast-but-incredibly-fragile variant.