The Outbreak level worked well as an introduction. The first time my friend played and went into the inventory, as soon as a zombie grabbed her she threw the controller to the ground. Too funny.
Below Freezing Point, Hive and Hellfire had nice puzzles and crazy changes in atmosphere, I think I liked watching the cinematics than playing the levels. Although the blind Licker effect worked very well.
Decisions, Decisions was too much, between the university connected to a lab by a port underneath the overpass. And then there were sharks and hunters and bees and freaky zombies. Tried to cram too much into one level.