The fast ones. I remember in the Left 4 Dead demo, where you had to go down, and close the door to end the mission, I waited on the stairs. After a while, a really huge bunch of them came running towards me, all of my teammates were down, at the door there. I got so nervous, I ran away from them, and I was like, soooo scared. And then I ran through the door, behind my teammates, and started popping them.
But slow movies are awesome too, since they seem to do more of those in-your-face, jump-out-of-your-seat moments, and they're generally very silent, and hide theirselves, and have alot of magical powers.