I wouldn't exactly compare James Patterson to King or Koontz. Whitley Streiber (sp?) was similar to Dean Koontz at one time. Clive Barker is the British version of Stephen King. Thomas Harris & his "Hannibal" type books are similar to Dean Koontz. John Saul is like a cross between Koontz & King. Dan Brown is another good one sort of like a cross between Patterson & Koontz.
If you like to dabble more into the fantasy side of sci-fi, I strongly suggest Margaret Weis & Tracy Hickman. They have several different series that will keep you reading for a long, long time.