I like Great Train Robbery myself. Slick main character, cool setup
Second would be Terminal Man.
Third, Rising Sun
I read them around grade 6-8 as well, like others here. I have Timeline and Prey, but haven't read them yet.
Edit: I tried reading Grisham once. Got like 40-50 pages into The Firm, it did nothing for me. Haven't picked up one of his books since. I've read a little Koontz, though. No hurry to read more, as from all I've heard it's horribly derivative and just plays the same theme over and over.