I'm surprised nobody else has mentioned Strangers. This was the first Koontz book I ever read, and it was awesome. I'm not much of a Koontz fan, but a few of his that I really did like are:

Strangers (obviously :D)
Dragon Tears
The Bad Place
Twilight Eyes

And I seem to recall that I really liked the one about the superintelligent dog --- was that Watchers?? It was years ago that I read it, so I'm not sure if I've got the right title.

I haven't read any of his more recent work, so maybe he's gotten less predictable in the last few years? My mother-in-law just ordered Life Expectancy --- guess I'll have to borrow that one when she's done. Has anyone else read that one yet, and was it any good?