I know I might stand alone on this one, but The Attack seemed incredibly unimportant. The Ellimist just whisks them away in a random fight to save a species the yeerks might evolve into, which will eventually lead to no where in the story line. The Return did make me vomit in disgust from watching one of my favorite fictional characters of all time get utterly reduced to Visser 3 one dimensionalism.
Most important book...hmm Perhaps the Predator because it still was setting the stage for an unknown book series and set a MAJOR story line. I may also choose The Underground because thats the first time I can really remember them seriously questioning ethical dilemmas. Yeah don't kill people they always had, but wondering whether they should force people to live with a braindead yeerk or try to free them was the first in a long history of self doubt, at least that I can remember.