I adored 28 Days Later for its ability to disturb.
Scenes like the reminiscence of the 20,000 people at the airport, and the crayon picture of a child's father being shot, all awesome.
I wrote my 10,000 word film assignment on this film, and by god it was worth it just to understand the true depth of this movie.
I especially liked the un-american quality of it. "Oh s--t, Zombies! Quick, everybody break out the M16s from the closet and quash the invasion in approximately 30 seconds."
The infected were cool because they weren't invincible, slow, and lethargic, but fast, bloodthirsty, delerious killing machines capable of being killed by a quick blow to the head like a normal human being.